The z-domain model of digitally controlled Buck converter is derived by only considering the effect of sampler-and-holder. Based on the z-domain model, the cause of the low-frequency bifurcation is analyzed. The location and type of the bifurcation point when a system loses stability is predicted in terms of the system eigenvalues. Moreover, the frequency and the amplitude of the low-frequency oscillation, i.e., the wave equations, are deduced. These theoretical results are verified by PSpice simulations.