The mechanical seal of two diesel oil pumps in the ethylene cracking unit of a petrochemical company has leaked frequently since 2019. Up to 10 repairs have been carried out due to mechanical seal leakage. The service life of the mechanical seal was far from that required by API682. In this paper, the causes of frequent leakage of the mechanical seal of diesel oil pumps are analyzed. It is found that the main causes of mechanical seal leakage include the carbon deposition in bellows, the damage and fracture of dynamic and static rings and the failure of auxiliary seal rings. Some corresponding improvement measures are taken to improve the operation safety and reliability of mechanical seal and prolong the operation cycle of pumps.