A power supply circuit for a CPU includes a first control circuit, a switch circuit, an operational amplifying circuit, and a second control circuit. The first control circuit is connected to a BIOS to receive a CPU identification signal. The switch circuit is connected to the first control circuit, and also connected between a CPU and a PSI pin of a VRM. The operational amplifying circuit is connected to the VRM to receive a current monitor signal output from the VRM. The second control circuit is connected to the operational amplifying circuit, the first control circuit and the PSI pin of the VRM. When the CPU identification signal is at a high level and the current monitor signal output from the VRM is lower than a reference voltage of the operational amplifying circuit, the switch circuit turns off, and the operational amplifying circuit outputs a second control signal to make the PSI pin of the VRM grounded.