Abstract: The mechanical seal in nuclear coolant pumps is important to the safety of nuclear facility. The mechanism of second stage mechanical seal in hydrostatic seal system of nuclear coolant pumps is studied using the mixed lubrication model and fluid-solid interaction model. When the first seal loses efficacy,the second seal is hydrostatic seals. The pressure difference of water,about 15.5 MPa,causes the rotating ring has a convergent radial taper while the deformation force of elastic ring decreases the radial taper. The face seal has a relatively small convergent radial taper,about 1 300.9 μrad. The leakage rate is 1 867.8 L/h when the inlet temperature is 65℃. The thickness of elastic ring is bigger,the leakage rate is lower. The inlet temperature of water and the friction performance of the material affect the performance of the second seal as well. The second seal is contacting face seal under pressure 0.17 MPa when the first seal is in normal operating conditions,its leakage rate is 11.4 mL/h,and temperature rise of water is 23.9℃.