A microreactor competent to control the microprecipitation environment is exploited to investigate the direct precipitation process between iron and phosphorus sources. The dependency of the reaction species on the purity of the final products is revealed. The Fe3+ hydrolysis and the pH of the mixture are confirmed to have decisive influences on the generation of byproducts. The addition of phosphoric acid in the iron source is verified to be an effective method to simultaneously inhibit Fe3+ hydrolysis and adjust the pH of the mixture. By optimizing the amount of phosphoric acid addition, pure nanoiron phosphate with an average size of ca. 20 nm is successfully prepared by a continuous precipitation process. Assuming instantaneous mixing and fast precipitation reactions, Fe3+ and HPO42- are indicated as the main species during the pure iron phosphate precipitation. The criteria would lay a practical guidance for achieving a continuous synthesis of nanoiron phosphate with high purity.