Abstract: The influence of background stray light can not be ignored in the high precision PST test. In order to improve the precision of the PST test, a high performance light trap system which can effectively suppress the background stray light was developed. The quantitative mathematical relationship between the main design parameters and the test error of PST was given based on the theory of the PST test and the theory of optical radiation transfer. The design idea of the light trap was proposed, which was fully controllable in the scattering path and greatly increased the scattering times to the imaging field of view. Through the simulation comparison experiment of some light trap models, the superiority of the design ideas and the model was verified. The result show that, when a light trap with diameter of 20 m is used for the PST test to an optical system with diameter of 2 m and size of 2.8 m3.5 m11 m, the testing error can be as small as 1.4910-10, it is reduced by about 4 orders of magnitude compared with the test system without light trap under the same experimental space, and reduced by about 2 orders of magnitude compared with the existing light trap schemes under the same experimental space. The light trap can be used to the high precision PST for the large space telescope.