A combined toolbox, including: a plurality of first side walls, each of the plurality of first side walls including a plurality of connection projections; a plurality of second side walls, each of the plurality of second side walls including a plurality of connection recesses, adjacent said first and second side walls being connected by engagement of the plurality of connection projections within the plurality of connection recesses, the plurality of first side walls and the plurality of second side walls defining a receiving space; a plurality of fastening assemblies, each of the plurality of fastening assemblies including a bolt and a nut; wherein one of the adjacent said first and second side walls (10, 20) further includes a plurality of through holes, and the other of the adjacent said first and second side walls (10, 20) further includes a plurality of receiving holes which respectively correspond to the plurality of through holes; characterized in that each of the plurality of receiving holes receives one said nut, each said bolt is disposed through one of the plurality of through holes and within one of the plurality of receiving holes and is screwed with one said nut, and the bolt and the nut of each of the plurality of fastening assemblies fixedly secure the adjacent said first and second side walls (10, 20); wherein two opposing sides of each said nut are respectively abutted axially against an inner face of one said receiving hole, and another two opposing sides of each said nut are respectively abutted radially against the inner face of one said receiving hole; wherein a distal end of each said bolt is abutted axially against an inner face of one said receiving hole; wherein each said connection projection includes a necked portion and an enlarged portion connected with the necked portion, each said connection recess includes a necked opening and an inner room in communication with the necked opening, each said necked opening includes a narrowed section and two widened sections respectively connected to opposite sides of the narrowed section, each said enlarged portion is disposed through one said necked opening and blockably engaged within one said inner room, each said necked opening restricts one said necked portion, each said narrowed section restricts one said necked portion; and each of two opposite sides of each said enlarged portion includes a lateral flange; wherein each said connection recess further includes an insertion opening whose extent is at least larger than an extent of the lateral flange of the connection projection so that the connection projection can be inserted, via the insertion opening, into the connection recess (21,) and slides so that the lateral flange of the connection projection can be reliably blocked at the necked opening of the connection recess. a plurality of first side walls, each of the plurality of first side walls including a plurality of connection projections; a plurality of second side walls, each of the plurality of second side walls including a plurality of connection recesses, adjacent said first and second side walls being connected by engagement of the plurality of connection projections within the plurality of connection recesses, the plurality of first side walls and the plurality of second side walls defining a receiving space; a plurality of fastening assemblies, each of the plurality of fastening assemblies including a bolt and a nut; wherein one of the adjacent said first and second side walls (10, 20) further includes a plurality of through holes, and the other of the adjacent said first and second side walls (10, 20) further includes a plurality of receiving holes which respectively correspond to the plurality of through holes; characterized in that each of the plurality of receiving holes receives one said nut, each said bolt is disposed through one of the plurality of through holes and within one of the plurality of receiving holes and is screwed with one said nut, and the bolt and the nut of each of the plurality of fastening assemblies fixedly secure the adjacent said first and second side walls (10, 20); wherein two opposing sides of each said nut are respectively abutted axially against an inner face of one said receiving hole, and another two opposing sides of each said nut are respectively abutted radially against the inner face of one said receiving hole; wherein a distal end of each said bolt is abutted axially against an inner face of one said receiving hole; wherein each said connection projection includes a necked portion and an enlarged portion connected with the necked portion, each said connection recess includes a necked opening and an inner room in communication with the necked opening, each said necked opening includes a narrowed section and two widened sections respectively connected to opposite sides of the narrowed section, each said enlarged portion is disposed through one said necked opening and blockably engaged within one said inner room, each said necked opening restricts one said necked portion, each said narrowed section restricts one said necked portion; and each of two opposite sides of each said enlarged portion includes a lateral flange; wherein each said connection recess further includes an insertion opening whose extent is at least larger than an extent of the lateral flange of the connection projection so that the connection projection can be inserted, via the insertion opening, into the connection recess (21,) and slides so that the lateral flange of the connection projection can be reliably blocked at the necked opening of the connection recess.