A controlling circuit for automatically adjusting clock frequency of a CPU is provided. The controlling circuit includes: a current sensing circuit for converting a current signal of the CPU to a voltage signal; a voltage amplifying circuit for amplifying the voltage signal; a multi-stage switching circuit for converting the amplified voltage signal to switched signals; and a priority decoding circuit decoding the switched signals, the decoded switched signals being input to a clock generator of the CPU, and thereby adjusting the clock frequency of the CPU to fit different load on the CPU.