Abstract: Focusing on the power system of single hydropower unit with enhanced dead zone, this paper analyzed the influence of load parameter on ultra-low frequency oscillations (ULFO) by the phase portrait method and non-smooth bifurcation theory, and revealed that one of the mathematical mechanisms of ULFO is C-bifurcation. In detail, firstly, the boundary equilibrium bifurcation (BEB) and non-smooth bifurcation theory of the sub-regional smooth system were introduced, illustrating that one of the reasons for the oscillation of general dynamic systems is the non-smooth bifurcation associated with switching between different smooth systems. Secondly, a simple mathematical model of the power system of single hydropower unit with enhanced dead zone was given. And then, the bifurcation characteristics of the system equilibrium point from existence to absence under different load disturbances were analyzed. Finally, based on the time domain and phase portraits, the dynamics of the system after disturbance were discussed under different load disturbances with/without enhanced dead zone. It shows that except for the absence of system equilibrium points, the system trajectory converges to a wide range of limit cycle, corresponding to the C-bifurcation of load parameter, which is one of the mechanisms of system ULFO.