When a large aperture collimator is used to test large aperture optical devices or components, the error introduced by the collimator will effect the measuring accuracy. This paper proposes a new method to eliminate the error from optical test results. The method uses an interferometer to obtain wavefront information of the collimator and the optical testing system, and adopts 37 Standard Zernike Phase polynomials to fit the wavefront. The error introduced by the collimator is then separated by subtraction of two sets of coefficients and the true wavefront information of tested optical system is obtained by simulating the light cone of a real optical system using the simulation model of tested optical system established in ZEMAX. The feasibility of this method in large aperture optical testing is verified by using the optical system model in ZEMAX, and an experiment is performed by using a small collimator with a focal length of 1 597 mm and an aperture of 150 mm, as well as a camera lens with a focal length of 50 mm. The experiment results show that the calculated wavefront P-V error is less than 0.005λ and the RMS error is less than 0.001λ after eliminating collimator error as compared to the real tested optical system wavefront. It meets the accuracy requirement of testing optical systems for image quality parameters in a laboratory.