In buck converter, in order to obtain high efficiency under different load conditions, pulse width modulation (PWM) is often used under heavy load, and pulse frequency modulation (PFM) is used under light load, so the mode switching signal is needed to control the working state of the whole buck converter. At the same time, the mode switching signal can also be used to adaptively change the grid width of the power stage circuit, which reducing the grid capacitance of the power tube and improving the efficiency of the whole circuit. An adaptive peak current mode switching circuit was designed to generate a mode switching signal. Its principle is to monitor the change of peak current and generate a peak voltage. After comparing the peak voltage with a reference voltage, the mode switching signal is obtained to decide whether the buck converter adopts PFM mode or PWM mode. The simulation results show that the circuit can switch smoothly between the two modulation modes in the range of load current from 0.5 mA to 500 mA, and its peak efficiency can be improved to over 94%.