Abstract: To evaluate the performance of large telescope under different kinds of error source, the normalized Point Source Sensitivity (PSSn) was introduced, which is firstly studied by the group of Thirty Meter Telescope(TMT) team to balance all the deviation of the telescope and also budget the error. First and foremost, character of the normalized PSSn was studied specifying the advantage in the evaluation in all the frequency domain. Then, the PSSn was compared with the traditional metric(FWHM, Strehl ratio, MTF, etc.). And the multiplication property was discussed. Using of Zernike polynomials allowed linking PSSn to the static error faced by large telescope, and the dynamic one was linked to PSSn by MTF. Lastly, the method was applied to a large telescope. Under mechanical vibration, the PSSn of this telescope main mirror decreased from 0.996 to 0.991. It aimed to guide beginner in the field of large telescope in the error analysis field.