Multi-grid method is used to solve the 3D electrostatic field distribution problem. The results to calculate concentric spherical system and len of bi-cylinder-cathode indicate that computational accuracy is 10^-3～10^-4. Comparing the two results in view of computational efficiency and computational accuracy , it appears that MGM is superior to FDM in solving electrostatic field distribution for the electron optics problem and it can greatly improve the computational efficiency of field distribution. A firm ground is provided for the design of non-rotation symmetry tube.