Abstract: In safety research of nuclear power plant, the reliability research of thermal hydraulic passive system bases on the numerical models. The models are always very complex. They have many input parameters with uncertainty. The influence of the parameters on the uncertainty of the model’s output is quite different. The goal of the sensitivity analysis is ranking the parameters’ influence on the uncertainty of the model’s output and finding the key parameters. In the paper the methods of sensitivity analysis were described, the rank transformed regression method to analyze the parameters’ sensitivity of the residual heat remove system model in HTR-10 was applied, and the key parameters were found and the model was simplified. Further more, the failure probability of the simplified model was calculated by surface response method. The result from the simplified model is very close to that from the original model.