Abstract: Considering the single-hydropower-generator power system (SHG-PS) with dead zone and limit in the case of negative damping, this paper analyzed the ultra-low frequency oscillation (ULFO), with respect to the sudden load disturbance and the non-smooth bifurcation theory of the piecewise-smooth continuous system (PSCS). It can be revealed that one of the mechanisms of ULFO is the switching- type oscillation, which is corresponding to the single crossing Hopf-like non-smooth bifurcation. In detail, first, the boundary equilibrium bifurcation (BEB), the Hopf-like bifurcation and the single crossing Hopf-like bifurcation of the PSCS were introduced. Secondly, the non-smooth dynamic model for the SHG-PS was presented. Then, the difference of frequency oscillation with/without switching boundary of dead zone and limit for the SHG-PS engaged with negative damping was analyzed and compared. It shows that the stable continuous oscillation is related to the switching boundary, which cannot be simply explained by the negative damping theory or the traditional Hopf bifurcation. Furthermore, the analysis of the generalized Jacobi matrix shows that the switching-type oscillation induced by switching boundary of dead zone or limit, corresponds to the single crossing Hopf-like bifurcation. Finally, the non-smooth bifurcation characteristics of the equilibrium point and the trajectory after disturbance were analyzed with different sudden load disturbance. It shows that the single crossing Hopf-like bifurcation is a kind of Hopf-like bifurcation with a stable node crossing the boundary and becoming an unstable focus.