The surface accuracy of lithography projection objectives is nanometer scale, so the accuracy of the testing interferometer is also needed on nanometer to sub nanometer scale. In the field of high-accuracy optical testing, in order to ensure the measuring precision, the accuracy of the interferometer reference spheres mirror should be higher than λ/40. A kind of new mechanical structure of reference spheres mirror was designed. The reference surface deformations of interferometer reference mirror were computed by means of finite element method under gravitational conditions. In that case, maximum Peak to Valley (PV) value is only 4.88 nm and the maximum Root Mean Square (RMS) value is 1.04 nm. At the same time，The PV and RMS surface figure error of reference Spheres caused by ambient temperature fluctuation was achieved. Lens system Modulation Transfer Function (MTF) was calculated by using lens shape Zernike coefficients to evaluate the image quality under gravitational conditions. Results demonstrate that the designed mechanical structure of reference spheres mirror can satisfy high demands of reference spheres mirror. At last, a compensated mechanism driven by difference bolt was designed to compensate some errors.