Abstract: The main shaft sealing rubber blocks of unit No. 3 in Gongzui Hydropower Station have burned several times in recent years, and the phenomenon of these failures is slightly different. According to the two typical burning accidents, 4 possible causes of the failure were analyzed and one by one excluded, then the blockage of water seal rubber is determined as the main reason. Analysis results show that special unit location and heavy sediment in Dadu River have resulted in the blocking. Based on this, a series of treatment measures such as replacing shaft sealing materials and adding filtration system of lubricating water are proposed. Practice shows that these treatment measures are proper and effective.