Abstract: As one of the important environmental elements, groundwater is a kind of valuable natural resources. With the development of social economics, the requirement for groundwater is more larger than ever in the cities of the northern China. Because of drop of groundwater level arised from the excessive exploitation, the groundwater quality is faced to be polluted. Therefore, how to determine the drop value of groundwater level scientifically is of importance for the protection of groundwater quality. In practice, the drop value is often calculated using a certain model, which neglects the uncertain characteristics of hydrogeological parameters. In fact, as many other things, the groundwater resources system is also a system with the characteristic of unascertainty. On the basis of defining unascertained parameters of hydrogeology, an unascertained model for forecasting drop value of groundwater level is established in the paper. According to the model, not only the probable drop value interval of groundwater level but also its corresponding distribution of faith degree can be obtained. The study result shows that these models proposed above are suitable for risk assessment of groundwater level drop, and the research achievements also overcome the defects of traditional evaluation method effectively. In order to reflect the probability of drop value of groundwater level exceeding a certain standard, an unascertained event is defined. On the basis, a model for calculating the faith degree what the unascertained event happens is provided.