The invention relates to an adjustment circuit for controlling the turn-on timeand belongs to the technical field of electronic circuits.Areference voltage generating circuit is used for generating arequired reference voltage. In a voltage sampling circuit, a control end for controlling the minimum turn-on time of the voltage sampling circuitis controlled to be suspended or connected with a ground level or an off-chip resistor, so that different signals are generated to be connected with a turn-on time control circuit. Therefore, a transmission gate in the turn-on control circuit is enabled to output different voltage values, and then an output signal of the turn-on control circuit is generated after the treatment of a filter composed of a first capacitor and a first resistor. The minimumturn-on time control circuit generates voltage control signals with different conduction time periods according to the feedback of the output signal and the external enable signal of the turn-on control circuit, and the output signal of the minimum turn-on time control circuit. According to the invention, the turn-on state and theturn-off state of a switch tube in a switching power supply can beaccurately controlled. The influence problem of the process deviation of resistors on the turn-on time is solved.