A digital control buck converter based on adaptive frequency DPWM was proposed for the use of fixed-frequency voltage-mode PWM control. In the load step response, the output value of the DPID changed to modulate the duty cycle of the PWM signal, and the DPWM frequency varied according to the output error value, which improved the modulation intensity of PWM signal. The change of DPWM frequency was realized by fractional frequency division and detection of ADC output. The transient response of the circuit was improved effectively by subsection regulation. The buck converter was designed in a 018 μm CMOS process. The simulation results showed that when the load current varied from 10 to 20 A, the overshoot voltage was reduced by 5 mV, the recovery time was shortened by 105 μs, the undershoot voltage was reduced by 8 mV, and the recovery time was shortened by 96 μs.