Abstract: Groundwater is an important water supply source for the northen cities in China. Because of without scientific evaluation and management on groundwater withdrawal in the past years, the phenomena of over-mining of groundwater resources are universal in these cities. In groundwater allowable withdrawal evaluation, the traditional method is based on determination model. In fact, there are some uncertainties in groundwater system. Based on the characteristics of objective randomness and subjective uncertainty of it, the blind number theory in unascertained mathematics is applied to groundwater resources evaluation. On the basis of defination of hydrogeological parameter blind numbers, a model for groundwater allowable withdrawal calcualtion is proposed. Using the model established, not only the possible interval values of groundwater supply but also their corresponding faith degree can be obtained. According to these useful information, the risk results of groundwater allowable withdrawal evaluation can be carried out. Case study shows that the results derived from the unascertained mathematics theory accord closer with practical situation than that of traditional method of groundwater allowable withdrawal calcualtion.