In order to solve the problems of aligning the space optics by wavefront errors or stellar image, the large amount of calculation and the great influence of surface shape errors, and so on, a discrete light source is proposed. And based it on, an alignment and measurement optical system is designed. The optical system is designed to measure and align a Ritchey Chretien telescope quickly by using the discrete light source as the reference ray. When the parameters of the optical system have been known, the relationship could be established exactly between the alignment errors and the spot diagram on the image plane. A simulation flat form is developed by using of the optical design software and the mathematics calculation tool. Then, the algorithm is proved in the simulation flat form. The simulation results show that using the discrete light source could reduce the amount of calculation, decrease the effects of the mirror errors and the stray light and have the precise calculation result.