The invention discloses a soft start circuit for a switch power supply, comprising an oscillator, a fractional frequency counting module, a resistance network, a current sampling module, a current source I2, a resistance R100, a pulse duration modulation comparer, a phase inverter, an RS trigger and a power PMOS pipe P1, wherein, output clocks of a reset signal and the oscillator are input to the send of the fractional frequency counting module and the RS trigger together, an output signal of the fractional frequency counting module controls the resistance network to obtain output voltage which is connected with a negative input end of the pulse duration modulation comparer, the other negative input end is connected with the error voltage, the positive input end is connected with output current of the current sampling module and is overlaid with the current source I2 to obtain the voltage through the resistance R100, the output end of the pulse duration modulation comparer is input to the R end of the RS trigger after being inverted by the phase inverter, and the inverted output end of the RS trigger is connected with the grid electrode of the power PMOS pipe P1. In the invention, the soft start effect is good, the circuit structure is simple and the chip area is saved.