Absolute measurement is an important method to improve the accuracy of the Fizeau interferometer. To achieve the high accuracy of the reference wave front in absolute spherical testing, the influence of environmental factors including temperature, gravity and clamping force was studied. The Gram-Schmidt fitting was used to fit the deformation of the reference surface in representation of the Zernike coefficients. The contribution of the thermal deformation by temperature change and the change in Zernike coefficients caused just by temperature change were compared. The model of the thermal deformation was created and the corresponding aberration mode of the reference surface was analyzed which make the separation of the deformation caused by temperature possible so that the test accuracy can be enhanced. The result indicates that the deformation of fused silica is smaller than that of K9 and zerodur on the whole while the deformation of zerodur is smaller than that of K9 and fused silica in just temperature change. When the temperature takes one degree change, the root mean square of the zerodur surface reaches 0.37 nm.