Abstract: <FONT face=Verdana>Purification effect of phosphoric acid by wet process (WPA) by means of ammonium-neutralization method was studied and appropriate crystal modifiers were selected by sifting.Study results showed that the purification rate of Al,Fe,and F ions were all over 98%,while purification rate of Mg ions was only 50% at the final neutralization point (pH=4.0);and the produced monoammonium phosphate（MAP）was small and fine needle-like crystals.As the crystal modifier,F ion could obviously decrease the influence of Al ion on crystallization,but it did not have obvious effect on Fe ion.Chelated crystal modifier,Chet-a could basically eliminate the impact of Fe ion.Combination of F ion and Chet-a could basically eliminate the impact of impurities on crystallization and crystal morphology could also be improved.Slenderness ratio of MAP product could reach 3.5∶1 and the quality could match the related Standard.</FONT>