The invention of spatial light modulator (SLM) promotes the development of aspheric surface test. SLM has the advantage of real-time and low-cost in comparison with the etching computer-generated hologram(CGH) plate. The pixel-structure of SLM, which includes the pixel pitch, gray-level number and black matrix, has great significance on the reconstructed wave-front quality. In this paper, the effects of SLM pixel pitch, gray-level number and black matrix are analyzed by Fresnel diffraction theory and computer simulation. In the simulation, a concave spherical wave-front with a radius of 1000 mm is generated by the SLM with different pixel pitch, gray-level numbers and aperture ratios, respectively. The results show that the quality of the reconstructed wave-front gets poorer as the pixel pitch increases, the gray-level number decreases or the aperture ratio reduces. This work can guide the selection of the SLM in the aspheric surface test.