To promote the development of crystallization technology for recovering salt from high salinity wastewater, the effect of organic impurity on crystallization of sodium sulfate was investigated by using phenol as a representative organic impurity. The effect of phenol on crystallization thermodynamics of sodium sulfate was evaluated by measuring solubility data of sodium sulfate in water in the presence of phenol. It was found that the existence of phenol could suppress the solubility of sodium sulfate in water. The effect of organic impurity on crystal nucleation was performed by measuring the metastable zone width (MSZW) and induction, time of sodium sulfate. Two models (self-consistent Nyvlt-like equation and Classical 3D nucleation theory) were used to analyze the experimental data. It was found that Classical 3D nucleation theory (3D CNT) can better explain the effect of phenol on nucleation. From both MSZW data and induction time data, it was found that the existence of phenol will apparently increase the interfacial energy gamma, which will result in higher nucleation Gibbs energy barrier and thus lower nucleation rate. Furthermore, the existence of phenol will increase the critical nucleus radius r* and the critical Gibbs energy Delta G*, which means that the formation of the nuclei will be more difficult in the presence of phenol. According to the above analysis, the possible mechanism of influence of organic impurity on crystallization of sodium sulfate was proposed.