The asynchronous interconnection mode between Yunnan Power Grid and China Southern Power Grid is planned to be put into practice in 2016. However, during the practical experiment of raising DC power in asynchronous mode, the frequency of Yunnan Power Grid oscillates in ultra-low frequency for a long period. In this paper, through detailed analysis of the experiment data, the governor systems of hydropower units are found to be the direct reason of the oscillation, and the mechanism of governor systems offering negative damping during the oscillation is analysed. Furthermore, off-line data simulation is carried out to reproduce the process of this practical experiment, the results prove the correctness of the theory, finally some preliminary recommendations are proposed for further work.