Giant Steerable Science Mirror( GSSM) is the tertiary mirror of the Thirty Meter Telescope( TMT).To evaluate the performance of the GSSM under the mechanical vibration,the Normalized Point Source Sensitivity( PSSn) is used to understand and characterize the optical performance degradation. First and foremost,the definition of the PSSn is shown and the calculation procedure of the PSSn at the work condition where the telescope aberration is much smaller than seeing is presented to reveal the relationship between PSSn and seeing.Then,the optical degradation due to vibration is achieved by the two methods: one is the statistics moment for the arbitrary froms of vibration and other one is spectrum method for wind load. After the theoretical presence,the simulation is processed. The multi-axis-accelerometers are used to achieve the motion of the mirror. After integrated to displacement signal,the measured signal is applied to the mathematical model of the GSSM and the optical performance degradation under certain mechanical vibration can be achieved. The PSSN flows from 0.996 to 0.994 under some dynamic vibration. And then,the wind load is applied to the mathematical model using spectrum method with the PSSN flows from 0.996 to 0.991. This paper can help the system engineers to predict the performance of the GSSM in various work condition. What is more,PSSn is able to be combined just by simple multiplication,so the optical performance degradation can be easily co-considered with other error sources. Giant Steerable Science Mirror (GSSM) is the tertiary mirror of the Thirty Meter Telescope (TMT). To evaluate the performance of the GSSM under the mechanical vibration, the Normalized Point Source Sensitivity (PSSn) is used to understand and characterize the optical performance degradation . First and foremost, the definition of the PSSn is shown and the calculation procedure of the PSSn at the work condition where the telescope aberration is much smaller than seeing is revealing the relationship between PSSn and seeing. Chen, the optical degradation due to vibration is achieved by the two methods: one is the statistics moment for the arbitrary froms of vibration and the other one is spectrum method for wind load. The multi-axis-accelerometers are used to achieve the motion of the mirror. After integrated to displacement signal, the measured signal is applied to the mathematical model of the GSSM and the optical performance de The PSSN flows from 0.996 to 0.994 under some dynamic vibration. And then, the wind load is applied to the mathematical model using spectrum method with the PSSN flows from 0.996 to 0.991. This paper can help the system engineers to predict the performance of the GSSM in various work conditions. What is more, PSSn is able to be combined just by simple multiplication, so the optical performance degradation can be easily co-considered with other error sources.