The influence of enhanced governor of hydropower generator unit on ultra-low frequency oscillation stability of power grid is studied. Based on the characteristic that the enhanced governor mainly modifies the dead zone of the primary frequency control, and by the usage of the governor model which considers the dead zone of primary frequency control, the describing function of primary frequency control’s dead zone of enhanced governor is deduced in which the offset value of the steady state frequency of power grid is considered. The theory analysis results of the influence of enhanced governor on ultra-low frequency oscillation stability of power grid are proposed based on the describing function, and are simulated and testified in the single generator with load system and Yunnan power grid. The results show that the influence of enhanced governor on the stability of ultra-low frequency oscillation of power grid is related to the factors including the stability of governor system without dead zone of primary frequency control, the offset value of the steady state frequency of power grid and the disturbance size and so on. When the offset value of the steady state frequency of power grid is equal to the setting value of the dead zone, the ultra-low frequency oscillation of the hydropower generator unit with enhanced governor will develop into a self-excited oscillation.