Aimed at the problem of mechanical seal leakage of pre-polymerization axial flow pump P200 occurred during the star-up of polypropylene device, the critical pressure of the pressurization piston was determined based on theoretical calculation. By combining analysis of the practice of the mechanical seal, an applied solution for perfecting the pressurization piston was found out. Moreover maintenance works were strengthened when the pump shutdown, especially before starting up the pump to avoid the impurity entering into the seal. The solution was proved by actual operation that it could effectively avoid the failure of mechanical seal, and make it easy to start the polypropylene device.