This paper focuses on critical hazard rainfall of valleys that only has water level records but no water flow records. After statistically analysing the water stage data and the rainfall data during floods of recent years, this paper gets the relationship between rainfall and floods. With this relationship and water level records of historical floods, the critical hazard rainfall of valleys is obtained. Furthermore, this paper estimates the risk levels of each critical rainfall by simulating submergence with submergence model. By contrasting the simulated result to history records of some serious floods, the rainfall threshold got from statistic is proved to be reasonable. Due to the clear submerging depth and blurry occurrence time of flood records, the rainfall threshold obtained still needs to be verified. Although the occurring time of the estimated hazards disaccords with reality, the low level risk rainfall threshold got verified in rainy season of 2012. It can be used to predict a low risk rainfall flood by lowering the risk critical rainfall reasonably. For other levels of risk rainfall, verification is very necessary once new flood examples are collected.