In order to get lower uncertainty,a method was proposed to compensate the gravity deformation caused by flipping in vertical absolute three-flat test. The gravity deformation of the flat surface was simulated by finite-element software and added to the three-flat test calculation. The calibrated results were compared with shift-rotation absolute testing method,and the root mean square of the surface map difference was less than 1nm without power term. For power term verification,a flat supported on different rings was tested and subtracted. For power term verification,a flat supported on different rings is tested and the difference between the tests and the simulation was less than 9 nm peak to vally,which was usually sufficient for flat power term testing. This experiment also proves the accuracy of the power simulation indirectly,the improved three-flat test can be applied in high precise flat absolute calibration including the power term in vertical direction.