Abstract: In collimating beam optical path, the error or minor movement of optical elements in optical system could be taken as minor rotation about a fixed point. The imaging relationship could be summarized as iterations of conjugating relationship between object and image in turn. The mathematical model of optical alignment and error analysis is built from the dynamic imaging method. The effect of beam splitter alignment error on optical communication system is analyzed with this model. The mechanical alignment of beam splitter system is theoretically supported with dynamic coordinate of every mirror. The axes parallelism between laser transmitting system and beacon receiving system reaches 5 after alignment.