Peak current controlled buck converter with current source load shows the complex phenomenon of fast-scale and slow-scale subharmonic oscillations. Its piecewise smooth switching model and discrete iterative map model are established in this paper. According to its discrete iterative map model, the effects of circuit parameters on the nonlinear dynamic behaviors of buck converter are analyzed by numerical simulation. It is found that attractive regions of fast-scale and slow-scale subharmonic oscillations and dual tori phenomena exist in the bifurcation diagram and Poincaré mapping, respectively. Time-domain simulation waveforms and phase portraits of buck converter are obtained by Runge-Kutta algorithm using the piecewise smooth switching model. The research results indicate that n-type subharmonic oscillation constituted by subharmonic oscillation and frequency-reduced subharmonic oscillation exists in the inductor current, and sine-type subharmonic oscillation constituted by fast scale and low scale exists in the output voltage, respectively. Analysis and simulation results are verified by experimental results.