Based on the piecewise smooth model, the smooth model and the discrete iterative model of proportion-integration (PI)-based voltage-mode Buck converter are derived. In this paper, it is proved that the chaotic attractor moves on the load line and is controlled by duty cycle, and that the manifold of the model moves around the chaotic attractor accompanied by the occurrences of period 1, period 2 and chaos phenomenon. The linear relationship between output voltage of the PI-controller and output voltage of Buck converter is derived, and then reveals that the proportional factor is a dominant one in PI controller. The period-doubling bifurcation, border collision and chaos are analyzed, and the state transfer process is exhibited. Experimental results verify that the theoretical modeling analysis and the simulation are correct.