Since the current industrial method for ammonium paratungstate (APT) production from ammonium tungstate solutions involves evaporative crystallization to remove the excess ammonium, which discharges large amounts of wastewater and consumes considerable energy, a novel method for preparing APT from ammonium metatungstate (AMT) solutions by adding ammonia was developed and investigated systemically in this study. Thermodynamic calculations indicated that the dominant tungsten-containing anions depending upon the solution pH were: metatungstate [H₂W₁₂O₄₀⁶⁻] at pH 2–4, paratungstate [H₂W₁₂O₄₂¹⁰⁻] and [W₇O₂₄⁶⁻] at pH 5–8, and tungstate WO₄²⁻ at pH > 8. The experimental results showed that the temperature, ammonia dosage, stirring speed, reaction time and different ammonium salts added affected the crystallization efficiency and crystalline morphology. The APT crystallization efficiency reached 91.4% with an average particle size of 492.2 μm under the following conditions: a temperature of 85 °C, a 1.6-fold stoichiometric ammonia dosage, a reaction time 5 h, a stirring speed 150 rpm and a C_(NH4)2SO4 1 mol/L. The crystallization mechanism involved the transformations of isopolytungstate ion from H₂W₁₂O₄₀⁶⁻ to the intermediate H₆W₁₂O₄₂⁶⁻ and then to the final H₂W₁₂O₄₂¹⁰⁻, which resulted in APT precipitation due to accumulation and combination of the H₂W₁₂O₄₂¹⁰⁻ and NH₄⁺ ions in solution. The crystallized product exhibited the two primary phases (NH₄)₆[H₆W₁₂O₄₂]·10H₂O and (NH₄)₁₀[H₂W₁₂O₄₂]·10H₂O at 40 °C and 55 °C, respectively, while (NH₄)₁₀[H₂W₁₂O₄₂]·4H₂O was formed at temperatures ≥70 °C. Results showed that (NH₄)₆[H₆W₁₂O₄₂] acted as an intermediate in APT crystallization from the AMT solution, which has not been previously reported. Compared with evaporative crystallization, this method for preparing APT product from AMT solutions by adding ammonia exhibits the advantages of (i) less chemical consumption, (ii) minor amounts of discharged wastewater, (iii) low production costs, and (iv) environmental friendliness.