The invention discloses a method of producing dihydric phosphate, which essentially comprises steps as follows: firstly, chloride is mixed with excessive phosphoric acid in order to obtain material liquid; secondly, the material liquid is added into the spray dryer to atomize into tiny droplets; thirdly, the droplets are instantaneously contacted and heated with as much hot air as is sufficient to enable double decomposition reaction between chloride and phosphoric acid to happen, so that hydrogen chloride gas escapes through the double decomposition reaction between chloride and phosphoric acid, and the remaining reaction products are hydrolyzed, the pH value thereof is regulated, and a dihydric phosphate product is obtained after cooling and crystallization. The production method has technical advantages that the shortcomings of prior art can be overcome, the spray dryer is adopted as a core reactor to atomize the material liquid into small droplets to be contacted with the high-temperature hot air, the specific surface area of evaporation is maximum, so that the escaping time of HCl and the water vapor is 3 to 8 seconds only, the reaction conversion rate reaches up to 97 percent, the cost of raw materials can be significantly reduced, the material liquid continues to be in and out, the reaction device is simple and practical, so that large-scale industrialization productioncan be achieved.