Alignment for radial Hartmann test system (RHTS) and optical testing system is analyzed. A new method of alignment based on the law of refection,is proposed. A laser is installed on a rotary arm of RHTS emitting beamlet to the measured optical system,while a CCD camera is used to record the tracing for reflected spots. In this way,the offset between optic axis and slewing axis can be figured out. The experimental results show that the proposed method is simple and efficient. This algorithm can also be used in the alignment of large telescope.