River system has certain uncertainty. Determination of design flow is crucial to the calculation of water environment capacity. Uncertainty theory was introduced into water environmental calculation in the paper. It took design flow as a unascertained rational number, and calculated the value and its probability in different situations. Taking Jinan section of Xiaoqinghe in Shandong Province as example, it calculated the interval value and corresponding credibility under 90% design frequency by using unascertained rational number to ascertain practical situation of its water environment capacity. The result can supply scientific basis for water environment treatment.