Abstract: In this article, we analyse the pn junction injected by deep level impurity based on an exact transmission-line equivalent circuit model. We put forward the sche mes of program by modifying the model according to different pn junction, conducting mechaaism under various kinds of working status. The input parameters are doping profile, SRH coefficients of recombination centers and geometric parameters of pn junction. The program can provide pn junction ac and do characteristics and inner information such as density of carriers. The experiments show that the simulation results agree quite well with one of the measurements.