To accurately evaluate the load-carrying capacity of a flyback switching power supply, under the premise that core saturation will not exist in the whole range of operating temperature of a high-frequency transformer used by power supply, an evaluation method for the load-carrying capacity of the flyback switching power supply is proposed. Starting from judging the working mode of power supply under the corresponding working voltage and load conditions, the maximum output power from the power supply is derived and calculated by combining the maximum working current on the primary-side of the high-frequency transformer. In the range of maximum output power, the primary-side peak current under one certain actual load-carrying power is calculated theoretically. According to the maximum working magnetic density of the high-frequency transformer under the highest operating temperature, the primary-side reference current is determined, which is further compared with the primary-side peak current to realize the evaluation on the rationality of load-carrying power of the switching power supply. At the same time, by using the method of theoretical calculation and joint simulation, the primary-side peak current, optimal load-carrying power of switching power supply and evaluation results of actual load-carrying capacity are obtained, and it is found that the theoretical calculations are in good agreement with the joint simulation results. In addition, a test platform for the load-carrying capacity of a switching power supply was established, and experimental results showed that the load-carrying capacity evaluation results obtained using the theoretical calculation and joint simulation were basically consistent with the test results, which verified the proposed evaluation method.