In this paper, based on the existing research, one innovative method for synthesizing poly(m-phenylenediamine) (PmPD) polymer using the chemical oxidation was presented, and the prepared PmPD was used for the treatment of chromium in wastewater. Then, the mechanism of PmPD in the treatment of chromium was studied. The results show that the best method for PmPD synthesis is the drop-by-drop method; 25 °C is the optimum polymerization temperature of PmPD and 5h response time is the optimal synthesis time of the polymer. At pH=2, the removal rate of chromium from wastewater by PmPD polymer reaches more than 80%, and the removal effect of PmPD(Na2CO3)2 on chromium is the best. At pH>7 (the alkaline solution), the PmPD polymer has almost no effect on the removal of chromium. With the same amount of PmPD polymer, the higher the temperature, the better the removal rate of chromium ion by PmPD polymer; the higher the initial chromium ion concentration, the lower the chromium ion removal rate.