For optical surface measurement with higher accuracy, a simple method for removing surface figure deformation due to gravity was adopted and its principle research and error analysis were conducted. Based on optical flat supporting case, properties of gravitational deformation of the tested sample was analyzed by finite element method (FEM). In order to verify high accuracy of FEM analysis, a method which could remove the original surface deviation of the reference and the tested from the test results was adopted. According to the method, difference between theoretical and experimental results represented by Root-Mean-Square (RMS) value was only 0.404 nm. The result shown that the FEM analysis is accurate enough and the surface figure deformation due to gravity could be removed from test results efficiently. The method discussed here could benefit the high accuracy optical measurement.