Abstract: On the hot standby condition of the residual heat removal pump in a nuclear power plant, the temperature at the measurement points of the mechanical seal water exceeds the alarm value. To deal with this issue, the root cause analysis（RCA） tree is established. Methods of CFD analysis and classical thermodynamics theory are implemented. The study found that in the radial position, the heat shield has greater effect on the temperature in the cooling chamber, which is ±0.458℃/mm. The cooling chamber size have the optimize value for temperature of mechanical seal water. And when the cooling water rate is less than 5 m³/h, it increase is obviously for this issue. Study found the sleeve clearance is the major factor, when the value change 0.1 mm, the temperature of mechanical seal water reduced 14℃. The ambient temperature increased from 16℃ to 45℃, the temperature rise 0.13℃.