In this investigation, solid-liquid equilibrium for the quaternary system sodium carbonate + sodium chloride + hydrogen peroxide + water was studied at a temperature of 293.15 K using the moist solid method. Four solid phases were formed in the quaternary system that correspond to Na(2)CO(3)center dot 10H(2)O, NaCl, Na(2)CO(3)center dot 1.5H(2)O(2)center dot H2O, and Na(2)CO(3)center dot 2H(2)O(2)center dot H2O. The solubility fields of four solid phases were observed from the phase diagram. Results indicate that the crystallization field of Na(2)CO(3)center dot 1.5H(2)O(2)center dot H2O is very large, the solubility of sodium percarbonate decreases with the increasing of sodium chloride content, and the product of sodium percarbonate can reach a high level. The solubility data and the quaternary phase diagram can provide the fundamental basis for the preparation of sodium percarbonate and optimize the preparing "wet process" and especially can serve as a guide for the preparation of sodium percarbonate with sodium chloride as the salting-out agent.