The invention relates to a method for synthesizing a magnetic separation g-C3N4-Fe3O4 photocatalyst through a water bath for sewage treatment. The method comprises the steps that a g-C3N4 photocatalyst is prepared by taking urea as a raw material through a pyrolysis method and then dispersed into a FeSO4.7H2O solution to prepare Fe3O4 through a thermal oxidation method, and Fe3O4 is directly attached to g-C3N4 to prepare the magnetic separation g-C3N4-Fe3O4 photocatalyst. According to the method, the problem that a g-C3N4 powder photocatalyst is difficult to recycle is solved. The prepared photocatalyst not only has the higher photocatalytic degradation effect on wastewater, but also has the good magnetism, the powder g-C3N4-Fe3O4 photocatalyst can be easily and efficiently recycled through an external magnetic field, and therefore reutilization is achieved. In addition, the advantages that the preparation method is simple, and the raw material is rich and easy to obtain are achieved.