Abstract: Due to the lack of in-depth research on the characteristics and input time of AC security load in engineering design, the emergency security diesel generator set of thermal power plant may have insufficient first loading capacity.This paper presents a method for checking the first loading capacity of diesel generator set on site.By analyzing the relevant parameters, logic starting conditions and protection setting values of the diesel generator and AC security load, combined with the test design and implementation of safety measures, the scenario of simultaneous loading of important AC loads when the diesel generator interlock starts successfully is truly simulated, so that the matching relationship between the diesel generator and the actual first loading motor capacity on the security section can be checked, It ensures that the emergency standby power supply in the security section can be put into normal operation when the auxiliary power is lost, and improves the reliability of unit operation.