Flyback converter has two working states: continuous inductance current mode (CCM) and discontinuous mode (DCM). Usually, the critical state of CCM and DCM is selected as the working point to design the compensation loop of flyback switching power supply. Although the compromise scheme can take into account two working modes, it has the problems of small bandwidth and poor dynamic response, especially under light load. To solve this problem, a flyback switching power supply with switchable working mode is designed. The loop compensation network is designed for CCM and DCM modes respectively. The load condition is judged by the primary side current of the transformer, and then the compensation network is switched to realize the switching mode of the power supply. A prototype flyback switching power supply with output power of 120W was designed, and the simulation and performance test of the prototype were carried out. The test results show that the power supply can not only switch the working mode according to the load, but also has a good dynamic response under light load. The effectiveness of the scheme is verified, which is consistent with the expected design results.