Abstract: Inflow volume of a reservoir acts as a key reference for formulating scientific water resource regulation plans, ensuring the safety of water conservancy facilities and exerting their economic benefits. By using the monthly precipitation and inflow data from 1977 to 2020 at three observation stations in the water source area of Huanglongdai Reservoir, the distribution characteristics and correlations between the precipitation and inflow water volume are analyzed. In addition, the diagnosis model of monthly inflow volume is established with linear regression method. The results show that there are significant monthly changes in the monthly precipitation and inflow water volume of Huanglongdai Reservoir. The highest precipitation is identified during April-June, and the inflow water volume peaks from May to July. Meanwhile, there is a certain lag relationship between the monthly inflow of water volume and precipitation. Except that the inflow in January is only significantly correlated with the precipitation of the same month, the inflow of water in other months is highly related not only to the precipitation of the same month, but also to the precipitation of the previous one to two months. According to the linear regression model between the inflow water volume and the precipitation in the same period as well as the previous period, the explained variance of each month ranges between 63.3% and 90.98%, showing a high fitting ability. Moreover, the fitting degree is closely related to the precipitation concentration degree. Months with high precipitation concentration degrees are prone to lower diagnostic results, while months with low precipitation concentration degrees are accompanied with higher diagnostic results. This lays the foundation for using precipitation to predict the inflow water volume of a reservoir.