Abstract: This paper presents an efficient hybrid method consisting of finite-difference time-domain (FDTD) method, transmission line(TL) equations, and interpolation techniques for the coupling analysis of double conductor lines located in free space and shielded cavity excited by ambient electromagnetic wave rapidly, which can realize the synchronous computations of electromagnetic fields and transient responses on the lines. The FDTD method is used to compute the electromagnetic fields surrounding the double lines, and the TL equations applied for the coupling of double conductor lines can be build based on the interpolation techniques. Then the scheme of FDTD method is utilized to discrete the TL equations to obtain the transient responses on the lines and terminal loads. Meanwhile, the influence of the distance between the double transmission lines on the responses of the lines is analyzed to master the coupling rules of the lines. The accuracy and efficiency of this hybrid method are verified by comparing with the FDTD method via two cases.