A combinative power device including an AC-to-DC module including a first hole and a second hole formed thereon and a DC-to-DC module including a main body, a first pin and a second pin, wherein said first pin and said second pin are disposed on said main body, said first pin including a first end and a second end, said second pin including a first end and a second end, said second end of said first pin being inserted into a first partial hole of said first hole, said second end of said second pin being inserted into a first partial hole of said second hole, said combinative power device characterized by comprising: said first hole including said first partial hole and a second partial hole, said second hole including said first partial hole and a second partial hole, widths or diameters of said first partial hole of said first hole and said first partial hole of said second hole being larger than widths or diameters of said second partial hole of said first hole and said second partial hole of said second hole respectively, said first partial hole of said first hole connecting one end of said second partial hole of said first hole, said second partial hole of said second hole connecting one side of said first partial hole of said second hole, wherein said first partial hole of said second hole includes a chamfered-edge circular hole, and said second partial hole of said second hole includes a semicircular hole, wherein said first partial hole of said first hole includes a geometric hole, and said second partial hole of said first hole includes a long curved hole; and widths or diameters of said first ends of said first pin and said second pin being smaller than widths or diameters of said second ends of said first pin and said second pin respectively, said DC-to-DC module being rotated relative to said AC-to-DC module, so as to cause said first end of said first pin to be slid into said second partial hole of said first hole and cause said first end of said second pin to be slid into said second partial hole of said second hole, wherein said second end of said second pin is chamfered-edge cylinder shaped, and said first end of said second pin is cylindrical. said first hole including said first partial hole and a second partial hole, said second hole including said first partial hole and a second partial hole, widths or diameters of said first partial hole of said first hole and said first partial hole of said second hole being larger than widths or diameters of said second partial hole of said first hole and said second partial hole of said second hole respectively, said first partial hole of said first hole connecting one end of said second partial hole of said first hole, said second partial hole of said second hole connecting one side of said first partial hole of said second hole, wherein said first partial hole of said second hole includes a chamfered-edge circular hole, and said second partial hole of said second hole includes a semicircular hole, wherein said first partial hole of said first hole includes a geometric hole, and said second partial hole of said first hole includes a long curved hole; and widths or diameters of said first ends of said first pin and said second pin being smaller than widths or diameters of said second ends of said first pin and said second pin respectively, said DC-to-DC module being rotated relative to said AC-to-DC module, so as to cause said first end of said first pin to be slid into said second partial hole of said first hole and cause said first end of said second pin to be slid into said second partial hole of said second hole, wherein said second end of said second pin is chamfered-edge cylinder shaped, and said first end of said second pin is cylindrical.