To fully analyze precision requirements of surface figure precision and pointing accuracy of plane mirror for the Giant Steerable Scientific Mirror (GSSM) system of this Thirty-Meter Telescope (TMT), system error model of GSSM needed to be established. In this paper, up-to-down distribution mode of system error was adopted. Technological indexes were understood and analyzed and converted to precision requirements of surface figure precision and pointer accuracy of plane mirror, according to standardized Point Source Sensitivity (PSSn) with its good features of synthesis and degradation. On the other hand, disturbance factors affecting system performance were analyzed, and influence of gravity print-through and atmosphere disturbance on PSSn was compared by using analysis result of finite elements for static error. System performance evaluation was conducted with systematic transfer function for dynamic error. Finally, Index requirements of all key components were proposed, and system error model of GSSM was established to satisfy system requirements of TMT as basis for GSSM system development. The experiment results indicate that seeing condition is good (r0<0.1 m), influence of seeing on PSSn was great; mirror print-through could only exert its functions on condition of good atmosphere seeing. With increase of vibration amplitude, system transfer function degenerated, and its PSSn would also reduce correspondingly from 0.996 to 0.992. It can be found that PSSn has special advantages as the next index of performance evaluation and error distribution of telescope, and its unique synthesis features and frequency domain calculation method can contribute greatly for telescope construction of the next generation.