The primary mirror surface figure precision is a key factor for the large ground-based telescope. In order to study the surface figure error caused by gravity deformations of primary mirror cell, and axial and lateral supporting system, the mirror cell and mirror supporting structures for a telescope were introduced. A precise finite element model of the primary mirror, mirror cell and mirror supporting structure was established by using the finite element method. Deformation of the primary mirror surface under supporting was calculated and the surface figure was tested by ZYGO interferometer. By contrast with the calculated results and tested results, it shows the mirror surface figure error caused by the supporting structures, and verifies the correctness of the finite element model.