The Multi-Channel Photometric Survey Telescope(Mephisto)is a wide-field ground-based telescope with a 1.6 m primary mirror and 2◦field of view(FOV).The telescope is capable of synchronous photometric imaging in blue,yellow,and red channels covering a wavelength range from 320 nm to 1000 nm by using large cubic splitters with dichroic coatings.A high-accuracy photometric survey requires higher requirements for the transmittance and reflectance of dichroic coatings.However,the light ray incident angle on the cubic splitter surface is 45◦±6 ◦, and the refractive index difference between the p-component and s-component of incident light causes an obvious p/s efficiency difference of both the reflection and transmission through the cubic,which will introduce extra error on the photometric accuracy.This paper proposes an effective method to calculate the uniformity of the imaging efficiency of the Mephisto.By sampling on the pupil plane with each FOV,the efficiency of each light ray can be calculated by the curve fitting method.The efficiency difference over the full focal plane is less than 2.5%,1.5%,and 0.5%for the blue channel,yellow channel,and red channel respectively,introduced by the cube dichroic coatings and the filters,which can be used for the calibration.The method paves the way to solve coating uniformity for large aperture telescopes.