The present disclosure relates to the technical field of ferric phosphate, in particular to a process and a system for preparing ferric phosphate by mixing and stirring in a reactor with external circulation. Disclosed is a process for preparing ferric phosphate by mixing and stirring in a reactor with external circulation, comprising the following steps: adding a ferric salt solution, a monoammonium phosphate solution, and a protective agent into the reactor, and subjecting the resulting mixed liquid to a preliminary reaction at 90-100° C.; after the completion of the preliminary reaction, carrying out an external circulation reaction at 85-90° C. The process described in the present disclosure has a high reaction speed, and the obtained ferric phosphate has a high yield and low content of impurity in the ferric phosphate.