Abstract: A novel Crank-Nicolson (CN)-FDTD method based on the split-step scheme is proposed in this paper, so as to calculate the electromagnetic transients in transmission line with complex circuit terminals accurately and efficiently. An analytical proof of unconditional stability of the method is provided. Combined with the hybrid one-port equivalent model, the transmission system is decomposed into lumped and distributed portions independently. It can solve the time response of the complex circuit networks by utilizing the Modified Nodal Analysis (MNA) method. Unlike the former methods, the maximum time step size is not limited by the restriction of Courant-Friedrichs-Lewy (CFL) stability constraint. In addition, the dispersion errors can be reduced by the precision sub-time-step scheme. The method is utilized to the transient analysis of the single transmission line. The results show that the proposed method provides higher efficiency and good stability under the same precision level.