Abstract: In order to meet the physical requirements, taper mechanism was applied to mechanical system design of in-vacuum undulator. This paper conducted a linear static analysis on taper mechanism, the key part of in-vacuum undulator developed by the Shanghai Synchrotron Radiation Facility, with finite element analysis software ANSYS Workbench, and obtained the distribution of deformation and stress and their effects on deformation and roll angle of the out-vacuum girder. The effects caused by taper mechanism were then minimized through compensating spring system to meet high accuracy at work. The analysis result was verified in the process of commissioning and is consistent with the actual situation.