In this work, the effect of temperature on the crystal growth rate of potassium sulfate in aqueous K2SO4 and KCl-K2SO4 solutions is analyzed. The crystal growth rate equation of K2SO4 is developed on the basis of the difference of chemical potentials of K2SO4 at the solid-liquid interface. The Lu-Maurer model is used to predict the activity coefficient of K2SO4. The crystal growth rate of K2SO4 in aqueous K2SO4 solution decreases, then increases with increasing temperature, and reaches its lowest value around 303.15 K, which is in accordance with previous experimental results. Meanwhile, the optimal crystallization temperature of K2SO4 in aqueous KCl-K2SO4 solution is discussed. An effective method is found to increase the yield of K2SO4 by increasing the crystallization temperature to 333.15 K.