Abstract: The crystallization mechanism of H₃BO₃-NaOH-H₂O solution in NPP was studied. The influences of factors such as sodium boron ratio (K), pH value, boron concentration, shaking and impurity on the crystallization of solution system were researched. The key compositions of the solution in different K values were also studied. The results show that the key composition of the solution is boric acid when K<0.2(pH<7.5), and the crystallization temperature of the solution decreases significantly with the increase of K or pH. The key composition of the solution is pentaborate when 0.2＜K＜0.4(7.0＜pH＜8.5), and the crystallization temperature is higher than 5 ℃ for the solution of boron concentration below 25 000 ppm. The key composition of the solution is borax when 0.4<K<0.8(8.5＜pH＜11.0), and the crystallization temperature relatively increases. The key composition of the solution is sodium metaborate when K>0.8. In all of the experiments, shaking and impurity accelerate the crystallization of the solution. According to the comprehensive analysis of the experimental results, when the minimum ambient temperature of the NPP is 5 ℃ and the target control value of boron concentration is 15 000 ppm, the recommended K value is 0.20-0.25 and the optimal K value is 0.23 to avoid solution crystallization.