A melted material communicating device of an injection molding machine used to connect or disconnect flow passages in a second injection unit and a first injection unit, characterized in that, it comprises: an oil cylinder, the cylinder body of which being fixedly attached to the second injection unit and being spaced apart from the nozzle of the second injection unit by an interval, the piston rod of the said oil cylinder facing toward the nozzle of the second injection unit; a flow passage connecting block having a communicating cavity, the said flow passage connecting block being located within the interval between the said oil cylinder and the nozzle of the second injection unit, and the said flow passage connecting block being capable of moving linearly within the interval; the said flow passage connecting block being rigidly attached to the nozzle of the said first injection unit, and the communicating cavity of the said flow passage connecting block being communicated with the flow passage of the nozzle of the first injection unit; when the piston rod of the said oil cylinder is ejected, the said flow passage connecting block can be driven to move in the direction of the nozzle of the said second injection unit, so as to abuttingly engage the said flow passage connecting block with the outer end of the nozzle of the second injection unit to communicate the communicating cavity of the said flow passage connecting block with the flow passage of the nozzle of the said second injection unit, such that the flow passage in the second injection unit is communicated with the flow passage in the first injection unit; when the piston rod of the said oil cylinder is retracted and reset, the flow passage connecting block is capable of disengaging from the outer end of the nozzle of the second injection unit, so as to disconnect the flow passage in the second injection unit from the flow passage in the first injection unit. an oil cylinder, the cylinder body of which being fixedly attached to the second injection unit and being spaced apart from the nozzle of the second injection unit by an interval, the piston rod of the said oil cylinder facing toward the nozzle of the second injection unit; a flow passage connecting block having a communicating cavity, the said flow passage connecting block being located within the interval between the said oil cylinder and the nozzle of the second injection unit, and the said flow passage connecting block being capable of moving linearly within the interval; the said flow passage connecting block being rigidly attached to the nozzle of the said first injection unit, and the communicating cavity of the said flow passage connecting block being communicated with the flow passage of the nozzle of the first injection unit; when the piston rod of the said oil cylinder is ejected, the said flow passage connecting block can be driven to move in the direction of the nozzle of the said second injection unit, so as to abuttingly engage the said flow passage connecting block with the outer end of the nozzle of the second injection unit to communicate the communicating cavity of the said flow passage connecting block with the flow passage of the nozzle of the said second injection unit, such that the flow passage in the second injection unit is communicated with the flow passage in the first injection unit; when the piston rod of the said oil cylinder is retracted and reset, the flow passage connecting block is capable of disengaging from the outer end of the nozzle of the second injection unit, so as to disconnect the flow passage in the second injection unit from the flow passage in the first injection unit.