The invention discloses a method for making potassium sulfate by utilizing power-generation straw ash. The method comprises the following steps of making straw ash after power generation pass through a filter sieve, so as to remove a large-size solid substance in the straw ash; filling the straw ash into a reaction tank, adding distilled water into the reaction tank, heating an obtained first mixture to 80 to 90 DEG C, and agitating the first mixture for 30min, wherein the mass ratio of the distilled water to the straw ash is 2 to 1; making a straw ash solution pass through a filter press for filtration, so as to remove an insoluble object in the straw ash solution, adding a sodium sulfate solution with a concentration of 27 percent into the straw ash solution, and agitating and making an obtained second mixture react for 20min; sending the filtered straw ash solution into an evaporating pan, evaporating the filtered straw ash solution to remove water, then transferring a solution into the reaction tank, and cooling the solution to a normal temperature, so as to obtain a separated out white crystal. According to the method, the potassium extraction operation is carried out by utilizing residual straw ash after the power generation; a waste resource is reasonably utilized; the investment cost is low; the technical process is simple; the economical benefit is high.