The odd-even function method and the N-rotation method are two commonly used methods in absolute flatness test. Both of them use the odd-even function decomposition algorithm in the process of recovering the surface shape, but they use different methods in solving the odd-odd function. The two absolute test methods are compared. The finite element analysis of the gravity deformation of flat is added to the simulation of the odd-even function method and the N-rotation method. The experiments of the odd-even function method and the N-rotation method are carried out on the flatness with a diameter of 100mm, and the surface shape recovery results are compared with the measurement results of the liquid surface method. The results show that the PV difference of surface shape restored by odd-even function method is 4.864nm, and that of surface shape restored by N-rotation method is 1.853nm. The results show that the accuracy of the N-rotation method is better than the odd-even function method when there is gravity. The effect of rotation angle error on the odd-even function method and N-rotation method is analyzed by simulation. The results show that the N-rotation method is stronger than the odd-even function method in resisting rotation angle error.