off-axis optical system has a wide application in space optics and remote detective area. the high surface shape accuracy can be ensured with the development of advanced manufacture technique. so the only condition which limits the wide application of off-axis optical system is how to realize the precise alignment of it. based on a rc reflective optical system whose diameter is 400mm, the alignment method which combines the high resolution initial placement and computer-aided alignment is introduced. by designing a system which can measure the off-axis fabrication and off-axis angle precisely, the high resolution initial placement of off-axis mirror can be ensured with a measurement accuracy of +/- 0.05mm and +/- 10 ''. the good initial placement can give a good initial state, so the computer-aided model can be converged well. the experiment shows that a system which has a good initial placement could have a good wave aberration of 0.04. after three times iteration adjustment.