Abstract: The negative damping effect of power system often leads to low frequency oscillation when the power fluctuation frequency of transmission line is generally between 0.2 and 2.0 Hz. Low frequency oscillation will cause over-current tripping of tie line or out of step protection and splitting between systems or between system and unit, seriously threatening the power system stability. Causes of low frequency oscillation include electrical, mechanical and hydraulic factors. Power system stabilizer (PSS) of excitation system can effectively suppress low frequency oscillation caused by electrical factors. Through analyzing low frequency oscillation of Unit 3 in Ertan Hydropower Station, it was founded that the oscillation was caused by hydraulic factor, which was solved by adjusting the load to overcome the vortex. The causes of low frequency oscillation of Unit 3 in Ertan Hydropower Station were analyzed and countermeasures for the system were put forward, which can provide reference for personnel in the same field.