Optical microscope is a common tool in histology and microanatomy, but it is difficult for traditional optical microscope to analyze a three-dimensional specimen. Adopting optical slices will overcome the three-dimensional effect. Optical slices are a series of two-dimensional images by moving the specimen along the focal plane axis. Managing every image with deblurring algorithms to weaken or wipe off the out-of-focus information of the neighbor slices. In this paper, a optical microscope imaging system is analyzed. The relations of defocus error, optical transfer function(OTF) and point spread function(PSF) with defocus distance of specimen-space ▵z are educed. And thick specimen imaging of this system is simulated, which is consistent with the real imaging process of this system. The results are useful for the study of deblurring algorithms.