Abstract: In the system of ammonium sulfate, magnesium and manganese double salt, the suspension density, crystal size, crystal number density and crystal crystallization kinetics of ammonium, magnesium and manganese double salt system were studied when the initial concentrations of Mn²⁺, Mg²⁺ and NH₄⁺ were respectively 28, 20 and 30 g/L, the rotation speed of evaporation bottle was 30 r/min, and the crystallization temperatures were 298, 308, 318 and 328 K. The results showed that the quality of crystal particles was quite good in the process of double salt crystallization when the crystallization time was 4 500 s with the crystallization temperature 328 K, the solution suspension density 240.4 kg/m³, the maximum average crystal particle size 633.982 μm, and the coefficient of variation 42.26. Meanwhile, the crystal nucleation rate B was calculated by fitting the crystal number density n₀ and crystal growth rate G with the ΔL law model when the crystal size L was higher than 50 μm. The fitting results were rather good since the crystal nucleation and growth kinetic equations were fitted by the optimized ΔL-law with correlation index R² above 0.98.