The invention discloses a method for recycling sodium sulfate waste liquid. According to the method, an industrial waste sodium sulfate solution is used, and a precursor of sodium ferrous sulfate can be obtained by steps of adjusting the concentration of the waste sodium sulfate solution, adding an appropriate amount of ferrous sulfate, performing stirring for a period of time at a certain temperature, and drying to remove moisture. A sodium ion battery cathode material, namely sodium ferrous sulfate, can be obtained by sintering the precursor at a certain sintering temperature. The invention finds a recycling way with a high economic added value for the industrial waste sodium sulfate solution, the sodium ion battery cathode material is prepared by using sodium sulfate wastewater as a raw material, the resource utilization of the sodium sulfate waste liquid is realized, good economic and environmental benefits are achieved, and the prepared sodium ferrous sulfate material has the advantages of high specific capacity, high working voltage, good cycling stability and excellent rate performance when used as a sodium ion cathode material.