Multimode fibers (MMFs) have been widely used in modern optical systems due to their ability to transmit multiple optical modes at the same time. However, external disturbances such as fibre bending and lateral stresses can sig-nificantly affect optical field transmission, leading to mode coupling and energy redistribution. In this paper, a robust method based on eigenmode decomposition and fourth order Ronge-Kutta (RK4) is presented to investigate the transmission of light fields. Using the RK4 method, the dynamic propagation of the optical field is simulated accurately by decomposing the optical field into orthogonal eigenmodes and solving the coupled mode equations. The simulation results indicate that the curvature radius and lateral stress have great effect on the mode coupling coefficient and the light field propagation. The proposed method effectively captures these complex dynamics. Compared to other methods, the proposed method is capable for the analysis of arbitrary input light field and the simulation set of modulation are more flexible. This work provides reliable modelling and optimization strategies for MMF systems, enabling enhanced performance in devices ranging from fiber-optic endoscopes to core components for advanced optical communications, signal processing, and photonic neural networks (ONNs).