The solubility data of KH2PO4 in the mixed solution of KCl and H3PO4 is basically required for designing and optimizing the preparation process of KH2PO4 from KCl and H3PO4 using solvent extraction. In this study, the solubility of KH2PO4 in KCl, H3PO4, and their mixture solutions has been measured respectively at temperatures of (288.2, 298.2 and 308.2) K. In KCl solutions, the solubility of KH2PO4 decreases as the concentration of KCl increases before KCl begins to precipitate. In H3PO4 solutions, the solubility of KH2PO4 increases as the concentration of H3PO4 increases. In mixed solutions of approximately equimolar KCl and H3PO4, the solubility of KH2PO4 decreases a little first and then increases remarkably with the increase of KCl (H3PO4) concentration. In the three systems studied, the solubility of KH2PO4 always increases with increasing temperature.