Abstract: This paper studies a novel hybrid method based on Finite Integral method software and transmission line (TL) equations for solving coupling problems of transmission lines in cavity in complicated electromagnetic environment. The software is convenient to build the model of arbitrary cavity without transmission lines and obtain electromagnetic field distribution in the cavity. Electric field probes are used in the software to exact the excited fields of transmission lines. TL equations are used to set up the coupling model of transmission lines in cavity. Then the excited fields are introduced into the TL equations as additional sources. The TL equations discretized by the difference scheme of finite difference time domain (FDTD) method are utilized to compute the current and voltage responses at terminal loads of transmission lines. The correctness of the hybrid method is confirmed by comparing with the results from references and obtained by the traditional method. In testing, the hybrid method in the simulation of coupling of transmission lines in electrically large and complex cavity has high precision and computational efficiency.