Fenton process is currently recognized as one of the most effective methods for treating organic wastewater. It is very important for the development and application of efficient Fenton process by exploring the reaction mechanism. Due to the complexity of Fenton's reaction, the mechanism behind it is still not fully explained, and given this, the reaction mechanisms of homogeneous Fenton, photo-Fenton, electro-Fenton and heterogenetic Fenton in removal of organic pollutants are reviewed. The purpose of this review is to help researchers and engineers gain a comprehensive and in-depth understanding of the mechanism of Fenton reaction in different systems, thus providing theoretical support for the development of Fenton reaction. Finally, the future development of Fenton process is prospected.