We implemented a stitching swing arm profilometer (SSAP) test for the inner and outer regions of a large aspheric surface with a short arm. The SSAP was more capable of improving sampling density of surface and was less sensitive to system error, like vibration noise and air-table noise. Firstly, a calculation model was built to evaluate the sampling density of the SSAP test. Then, sensitivity to system noise was tested when different lengths of arm were used. At the end, an experiment on a 3 m diameter aspheric mirror was implemented, where test efficiency was promoted, and high sampling density was achieved.