As a common structure of power electronic system, the unequal length multiconductor transmission lines model has a crosstalk problem which may affect the normal operation of the whole system. In this paper, the Finite Difference Time Domain(FDTD) format is employed to differentially discretize the transmission line equation, and the Kirchhoff’s law is utilized to process the connection positions of different multiconductor systems. Then, the iterative formulas of the voltage and current on the multiconductor transmission lines are derived, and the simultaneous solutions of all voltages and currents on the entire model are realized. Finally, the effectiveness of the method in analyzing the electromagnetic interference problem of electrically large transmission lines is verified by a numerical example.