The invention provides a method for preparing a PtM/C catalyst by composite electrodeposition, and belongs to the technical field of electrodeposition. According to the method, insoluble carbon carrier particles are added in a plating solution, stirring or ultrasonic vibration are performed, and a DC power supply is used for setting different currents and voltages to make the carbon carrier particles and the PtM alloy co-deposited on a cathode so as to form the PtM/C catalyst, wherein the bonding force between the carbon carrier and catalyst alloy metal is good, the particles are uniformly dispersed and the specific surface area is very large. The adopted composite electrodeposition method under stirring or ultrasonic conditions can not only make the nanoparticles of the PtM/C catalyst uniformly and stably loaded on the carbon carrier, but also has simple preparation process, mild reaction conditions and strong controllability.