This paper presents an efficient and systematic approach for transient analysis of hybrid interconnect systems. The approach uses the finite-difference time-domain (FDTD) method to fully characterize the subnetwork of the high-speed interconnects in the form of admittance parameters. It then uses the admittance parameters to construct a macromodel of the subnetwork by the vector fitting method (VFM). The resulting macromodel is ready to be synthesized into a SPICE-compatible circuit simulator to efficiently expedite the transient analysis of the hybrid system with interconnects and linear/nonlinear lumped circuit elements. Numerical examples show the validity of the method.