Two in-vacuum undulators have been developed for the first time in China at Shanghai Synchrotron Radiation Facility (SSRF), which has been opened to users since May, 2009. The precision of mechanical system is important to ensure performances of magnetic field. Efforts were made to manufacture in-vacuum undulator with high mechanical properties. The taper mechanism is adopted for the first time in the design of an in-vacuum undulator. A finite element analysis was performed to find out the deformation of out-vacuum girder and minimize the effect of linear rolling guide on it. In this article, the design and analysis results for the in-vacuum undulator are described.