During the practical asynchronous interconnection experiment, Yunnan Power Grid experienced an ultra-low frequency oscillation for a long period, which threatened the security and stability of China Southern Power Grid(CSG). In this paper, the effects of different hydro governor parameters and HVDC frequency limitation control (FLC) on the frequency stability are analyzed in a single-generator system and the BPA simulation system. From a large amount of simulation, the measures of optimally adjusting the dead zones and PI parameters of hydro governors are proposed, and the characteristics of different measures are compared. The proposed measures can effectively solve the ultra-low frequency oscillation problem in asynchronous interconnection mode of Yunnan Power Grid, which has been verified by the large and small disturbance experiments on the actual power grid.