In the case of a certain aircraft’s engine lubricating oil’s temperature drop is insufficient after turning on the lubricating oil heat diffusion ejector (hereinafter referred to as the ejector) under the condition of engine idling on ground in the hot weather, a reasonable flow analysis and relevant calculation method of ejector performance is given to solve this problem. Based on N-S equations and k-epsilon turbulence model, lubricating oil’s temperature drop before/after ejector under severe conditions is calculated in this paper. The results of the on-board ground test show that the calculation method is reasonable and numerical simulation results are consistent with the test results, which meets the requirements of general engineering calculations and provides a theoretical basis for the performance of an ejector. This paper also gives the method of performance improvement of an ejector based on the calculation.