Abstract: The structural of the ATE pump for Qinshan NPP 3# and 4# Units is analyzed, and the operation parameters and the design criteria of the mechanical seal is contrasted. Four factors that may lead to the seal failure which refer to too large seal face line speed and pressure and insufficient auxiliary flushing water flow and pressure are theoretically analyzed. It shows that the too large seal face pressure of the mechanical seal is the root cause which causes the mechanical seal burned. According to the conclusion, a set of re-mounted, balanced cartex-sn28/110-00 mechanical seal replaces the bulk-type and unbalanced mechanical seal M74N4/110-G6, which completely eliminates the frequently burned mechanical seal of the ATE pump.