The non-linear differential method was applied to the first-order and second-order differential appoximation. After non-linear grid dividing of radius and refractive index distribution of any multi-layer fiber,the numeralization of Helmholtz equation was obtained.By solving the characteristic matrix equation using Matlab,accurate mode field distributions of single mode fiber and multimode fiber were obtained,and the normalized mode field of five cladding dispersion flattened fiber was calculated.The results show that the non-linear differential method ensures the accuracy,and also improves the computational speed greatly,which lays a foundation for further study of other characteristics of optical fiber.