When the high-precision measurement of the curvature radius is performed, the measurement error is introduced under three-point support. The theoretical analysis and the finite element simulation of the errors are carried out and the simulation results are verified by experiments. The results show that the rotationally symmetric error of support deformation leads to the inaccurate radius. The compensation is less than 70 nm under the specific experimental conditions. Based on this, the measurement accuracy can be increased when the measured value of the curvature radius under three-point support is compensated.