The invention relates to a method for preparing iron oxalate, belonging to the field of chemical engineering. The method for preparing iron oxalate can recycle by-product of copperas generated when preparing titanium pigment with titanium concentrate. The method for preparing iron oxalate takes the by-product of copperas generated when preparing the titanium pigment with the titanium concentrate as raw materials, and comprises the following steps of: a. adding water to the copperas for dissolution to obtain a ferrous sulfate solution, adjusting pH value to be 5.6 to 6.5 by utilizing ammonia, filtering the solution; b. extracting the filtrate by utilizing P2O4, separating the filtrate to obtain an organic phase and a water phase; c. and adding a solution containing F to the water phase so as to deposit Mg<2+>, filtering the solution, adding an oxalic acid solution to the filtrate, and filtering the solution to obtain iron oxalate crystallization. The invention not only solves the problem of stacking the copperas, but also enhances the economic value of the copperas, provides a new choice for treating the by-product of copperas generated when preparing titanium pigment with a sulfuric acid method, and has wide application prospect.