Giant Steerable Science Mirror (GSSM) is largest flat mirror as the tertiary mirror of Thirty Meter Telescope (TMT).To evaluate the performance of GSSM under gravity load, the mirror figure needs to be tested. Erorr analysis is the most imortant step for the reliability of metrology. For its large size and plate character, Sub aperture stitching will be used to reach the full aperture figure. GSSMP is the prototype of GSSM. Research on GSSMP sub aperture testing error will help understanding and learning how to build GSSM. The stitching error is divided into two parts: rigid body location error and middle frequency turbulence. According to each part, theoretical analysis and experience have been done drawing the conclusion that the algorithm with probable marks will suppress rigid body error efficiently and the influence of the turbulence is achieved by smaller aperture system and interferometer. What is more, stitching order is also related to stitching order. Using one reference aperture and fitting other apertures onto it allow introducing least error to the final result. All the performance is specified by Slope RMS,which is required by TMT.After fitting, Shift error is 10－6 μrad, Piston error is 10－6 μrad, Tip/tilt error is 10－6 μrad and Air turbulence is 0.04 μrad. The error involved by the rigid body is ignorable, and the air turbulence is 3nm in RMS. By Slope MS, the reqiured metric of TMT, the influence of stitching error will be discussed and the orders chosen here did not increase the Signal to Noise ratio significantly in the figure at the level of 2%. Here the conclution was drawed that feature will help a lot in lowering the stitching error.