Abstract: Heat transfer model of inlet tube is established for Small PWR（ACP100） passive residual heat removal system. The heat transfer mechanism for the leakage of inlet line and passive residual system is researched. The results show that the temperature instrument located more than 0.3 m below the inlet line high point is evaluated sufficient to avoid the effect of the source. It is effective to detect and quantify the leaks by temperature monitor. Increasing the leak rate, the maximum temperature measured by the instrument tends to increase. For the leakage above 8×10^-4m³/h, the effect of axial heat flux can be disregarded without affecting significantly the calculation.