Star simulator acts ground calibration equipment of the star sensor, It testes the related parameters and performance of the star sensor. At present, when the dynamic star simulator based on LCOS splicing is identified by the star sensor, there is a major problem which is the poor LCOS contrast. In this paper, we analysis the cause of LC OS stray light , which is the relation between the incident angle of light and contrast ratio and set up the function relationship between the angle and the irradiance of the stray light. According to this relationship, we propose a scheme that we control the incident angle . It is a popular method to use the compound parabolic concentrator (CPC), although it can control any angle what we want in theory, in fact, we usually use it above ±15° because of the length and the manufacturing cost. Then I set a telescopic system in front of the CPC , that principle is the same as the laser beam expander. We simulate the CPC with the Tracepro, it simulate the exit surface irradiance. The telescopic system should be designed by the ZEMAX because of the chromatic aberration correction. As a result, we get a collimating light source which the viewing angle is less than ±5° and the area of uniform irradiation surface is greater than 20mm×20mm.