In order to solve the problems that the conversion efficiency of PWM DC-DC dropped sharply at light load, a PWM DC-DC circuit with a segmented output stage was proposed, which was used to optimize the conversion efficiency of light load. A load current detection circuit was introduced to sample and detect the output current. When the load was heavy, all power MOSFETs were output at the same time. When the load current was reduced, the power MOSFETs were turned off from the first stage to the second stage gradually, until the smallest size power MOSFET was only working in the lightest load. The simulation results showed that within a load range of 1~200 mA, the conversion efficiency of the proposed DC-DC converter changed more smoothly. At the light load of 1 mA, the conversion efficiency of the circuit was 81%.