In order to ensure the high measuring precision of large-aperture flat mirror, the accuracy of the largeaperture reference flat mirror must be controlled strictly. The influence of different distribution of adhered spots and different support schemes on the shapes of reference flat mirror are analyzed by means of finite element method (FEM) under gravitational conditions. A kind of support pattern of 300 mm aperture reference flat mirror is designed. The mirror is supported by 12 adhered spots, and the best supporting method of mirror frame by rotation compensation is researched. The Φ 300 mm reference flat mirror is assembled according to the analysis above. The assembly results indicate that the change of optical surface due to supporting is nearly 1.294 nm of root mean square (RMS), which coincides with the analysis and satisfies the requirement of shape precision of the large aperture reference flat mirror.