The present invention relates to a method of preparing wet process phosphoric acid, which comprises: dividing raw materials which contain phosphate rock slurry and sulfuric acid into two parts, and then feeding these two parts into two reactors separately. In the first reactor, feeding 70 90 wt. % of the total amount of the slurry and sulfuric acid, in which the ratio of phosphate rock slurry to sulfuric acid to recycled phosphoric acid is set to be 1:0.6.about.0.8:1.0.about.2.5. 10.about.30% of the reaction solution directly flows into conversion cell, participating in the conversion reaction of dihydrate gypsum. Thus resulted phosphoric acid concentration is 33.about.39 wt. %. The recovery efficiency of P.sub.2O.sub.5 is 99% or more. When putting coke powders in the conversion solution as filter aid, the filtration efficiency can be improved by 10.about.30%, and in so produced hemihydrate gypsum, P.sub.2O.sub.5 is 0.05.about.0.2%, water of hydration is 4.about.8%. The product can be used for producing sulfuric acid and cement without baking dry.