How to use surface water resources in an area is a complex problem.It is not simple to calculate the usable capacity of local surface water resources.This paper defined the usable capacity of the surface water resources in an area with Fujian Province taken as a case,and pointed out that the usable capacity can be divided 4 parts of usable capacity of local surface water resources,usable capacity of the water resources from the inter- province rivers,increased water capacity by the inflow diverted from other areas,and decreased water capacity by the outflow diverted to other areas. This paper also put forward a new method to compute the usable capacity of surface water resources in an area,and gave a reasonable conclusion for utilization of surface water resources in Fujian Province.