The invention discloses a method of preparing trisodium phosphate by using wet-process phosphoric acid. The method comprises the following steps: reacting carbomite with wet-process phosphoric acid to obtain an intermediate; and reacting the intermediate with sodium hydroxide to obtain a trisodium phosphate product. The method disclosed by the invention is short in process line, low in energy consumption, stable in product quality, low in production cost, convenient to operate and safe in production. Byproduct slurry can be further fully recycled. The whole production process is environment-friendly, clean and pollution-free, free of discharge of exhaust gases, wastewater and waste residues, responds to the policy of energy conservation, emission reduction and clean production, and overcomes the problems of complex process, unstable product quality, high energy consumption, environmental pollution and the like in the prior art. The purity of the obtained trisodium phosphate product is more than or equal to 98%.