Transmission lines are often employed in complex metal shielded cavities, where numerical algorithm-based simulations are memory-intensive and computationally inefficient. This paper proposes a novel method for simulating the electromagnetic transient response of transmission line load coupling problems, which combines an improved Locally One-Dimensional Finite-Difference Time-Domain(LOD-FDTD) method with Modified Nodal Analysis(MNA) to more efficiently and accurately solve field-line-circuit electromagnetic problems at fine structural gaps. Gaps and cables are treated as electromagnetic structures, and the improved LOD-FDTD method is employed to solve the field distribution; based on the circuit substitution principle, MNA is employed to analyze the port voltages and currents of circuit modules, thereby completing the hybrid calculation of transient electromagnetic response for the entire field-line-circuit coupling problem. Simulation results demonstrate that, compared to other analysis methods, the proposed approach substantially enhances computational efficiency while maintaining reasonable numerical accuracy.