Optical transmissivity parameter is an important parameter in evaluating the optic aiming device′s imaging luminance and clarity. This paper introduces an optical transmissivity detection system with a wide spectrum coverage and high precision for the aiming device, which is mainly applied to the production and test process of the aiming device. Focused on influencing factors on the measurement accuracy in the detecting process including polarization, beam displacement, stray light, spatial uniformity and nonlinearity of detector, quantitative analyses by verification tests or tracepro simulations and combinded standard uncertainty of highaccuracy measurement for spectral transmissivity are presented. According to the nonlinear error of detector, a correction is carried out by using the principle of least square method. The results of the calibration experiment have proved that the accuracy of transmissivity parameter in visible and infrared bands arrive at 0.35% and 0.65%, respectively.