In this article, we propose a method of recovering p-MePhSO3K from the industrial by-product of quizalofop-p-ethyl. The phase diagram of the solid-liquid equilibrium system of p-MePhSO3K2CO3-H2O at the temperatures of 15, 25, 35, 45, and 55 degrees C was determined and plotted. Based on the phase diagram of 25 degrees C, the separation and recovery process of p-MePhSO3K from the p-MePhSO3K2CO3-H2O system was designed by analyzing the crystallization fields. First, p-MePhSO3K and K2CO3 were crystallized from wastewater in turn through evaporation separation. Then, the crystallized p-MePhSO3K was purified further by washing with ethanol. Finally, the pure p-MePhSO3K was transformed into the intermediate p-toluenesulfonyl chloride and then the intermediate L-p-toluenesulfonate ethyl lactate for quizalofop-p-ethyl production.