Thermodynamics of Pb(II) complex equilibrium in Pb(II)-NH3-NH4Cl-H2O system were studied using the double equilibrium method. Varying the concentration of ammonia and ammonium chloride, respectively, in range of 0~5mol/L, the equilibrium concentration of all the species in the system are calculated, and thermodynamic diagram are plotted. The dissolution laws of Pb(II) in the system are revealed and the high dissolution fields of Pb(II) are found. The solubility of lead increase with the ammonium chloride concentration increasing, and the highest solubility reaches 0.29mol/L, whereas the concentration of lead always remains 0 in pure ammonia solution. Add ammonium chloride to the solution and maintain its concentration constant, the concentration of lead decreases with ammonia concentration increasing.