The utility model relates to a continuous production unit of phosphoric acid by wet process by -product alpha plaster of paris, the device include extraction system, conversion system, solid -liquid separation system and sulphuric acid dilution system, extraction system's export and conversion system's entry intercommunication, conversion system's export respectively with the entry intercommunication of filtrating system, lotion system, the export of filtrating system and extraction system's entry intercommunication, sulphuric acid dilution system and extraction system's entry intercommunication is passed through in the export of lotion system. This apparatus for producing utilizes the lotion to dilute the concentrated sulfuric acid, has eliminated direct the interpolations concentrated sulfuric acid to the heating load that the device led to the fact, has improved the production stability of product, and make full use of dilutes steam that the in -process produced and for the groove of changeing provides the reaction heat, has greatly reduced manufacturing cost.