For sustainable utilization of water resources, a reverse calculation method has been put forward, which considers water consumption of the water environmental capacity as the available water resources(AWR). This method is combined with the traditional forward calculation method, which is based on the amount of regional water resources. The factors that affect the AWR calculation, including the abandoned flood quantity, water demand in rivers, water qualification rate, water environmental capacity, and unit water consumption of pollutant discharge, are analyzed. The method was used to calculate the AWR in the Futunxi and Shaxi basins in the upper reaches of the Minjiang River and in the basins of the lower reaches of the main stream of the Minjiang River. The results show that the forward calculation method of the AWR should be adopted in the lower reaches of the Minjiang River. Similar amounts of AWR can be calculated by the reverse and forward calculation methods in the upper reaches of the Minjiang River, but the result of the reverse calculation method is smaller than that of the forward calculation method. Compared with the traditional forward calculation method, the combined forward and reverse method can take into account factors such as water diversion and storage in projects, water demand in rivers, and water quality, and assure sustainable utilization of water resources.