In order to alleviate the water supply–demand problem, a flood resource utilization strategy is proposed, called ‘Flood Utilization’. The strategy focuses on building large-scale water conservancy facilities and improving management measures. This paper presents the probability analysis of floodwater utilization in a confluence area, where a tributary joins a main river. Baicheng is used as the study area, where the Taoer River joins the Nenjiang River. After a large number of analyses, the main results and conclusions are as follows: First, the upper limit of available floodwater corresponds to the Taoer River's flood with a 5% probability of occurrence. Secondly, there are compensation characteristics between the two rivers which mean that the Nenjiang River can supply water to the Taoer River area. The analysis of monthly runoff, shows that there are compensation characteristics in 50.9% of the data period. The compensation rates (CRs) for the months from June to October are 0.2, 0.27, 0.25, 0.27, and 0.2, respectively. Thirdly, the differences in the runoff characteristics show that it is suitable for floodwater utilization. Finally, it is proposed that floodwater utilization measures are based on local conditions, such as the regional water storage characteristics and the runoff characteristics of the two rivers, and should be applied for different periods.