The microstrip circuit is mostly analyzed in transform domain, because its equivalent circuit equation is often nonlinear differential equation, which is easily analyzed in transform domain relatively, but hardly did in time domain, so the analysis of microstrip circuit is a hard work in time domain. In this paper, the FDTD method is used to analyze the microstrip circuit in time domain, by transforming the nonlinear differential equation into time domain iterative equation, selecting suitable time step, and having an iterative computing, the time domain numerical solution can be solved. The FDTD method analyzing the microstrip circuit provides a new way of thought for analyzing microstrip circuit in time domain.