Abstract: A method for controlling soft start of a power supply performs soft start under an open loop control mode after the power supply is started, enters a switching control mode when output voltage of the power supply reaches a reference value, starts to convert the output voltage of the power supply into digital data using an analog-to-digital converter for determining if the power supply is switched to a closed loop control mode and the output voltage is adjusted under the switching control mode. Using the foregoing technique can enhance the bit length utilization rate of the analog-to-digital converter, ensure a smooth waveform of the output voltage and suppress maximum excess or minimum deficiency or a transient response at the switching control mode.