Abstract: As to minimize size of the measurement system of large size and ultra-long focal length lens, two mirrors are used to change light path. In this case, installation error of the two mirrors added may be a large obstacle to accurate measurement of ultra-long focal length lens. For the sake of analyzing how installation error of two mirrors infect the angle of Moir stripes on receiver, Moir stripes are understood as parallel lines which composed of point light source with certain light-path vector. This approach proposed uses the calculation formula from mirror deflection angle to the reflector unit normal vector, and the matrix form of the law of reflection, to calculate the relationship matrix from vector of incident light to vector of exit light. Based on unit normal vector of two mirrors, incident light-path vector, exit light-path vector, and analytic geometry, equation of line on which image of point light source place can be presented, so we can get the angle of Moir stripes on receiver and so is the angle error caused by installation error of the two mirrors added into light path. In addition, the angle error of Moir stripes is calculated theoretically in 49 different situations of small mirrors deflection smaller than 1 degree. The results indicate that the installation error of the two mirrors should be taken into consideration. A new method by using autocollimator to adjust the installation error of the two mirrors is proposed, installation error of the two mirrors can be controlled to lower than 1 through this method. Moir stripes angle error should be better than(10-4) order of magnitude in this situation. Experimental results show that Moir stripes angle measurement error is 0.005 degree, which fully illustrates the effect of mirrors adjustment when this system is used for the measurement of ultra-long focal length.