Standard effective temperature (SET) is a commonly used index in thermal comfort evaluation. It was established based on two-node model which could reflect thermal regulation process of human body. The misunderstanding and confusion of SET are always existed as its definition and calculation method are relatively complex and have been revised several times since proposed. It is necessary to analyse and unify the application of SET index thoroughly. This study reviewed the development process of the SET, and interpreted its definition and calculation based on Gagge's 1986 paper. As the results, the correct definition and equivalent conditions of SET standard environment were proposed, and the modifications on calculation methods were conducted. It's pointed out that the clothing insulation and the air velocity of the standard environment were not of the fixed values but the functions of the actual metabolic rate. Explorations on calculation time length and adaptive items values of SET were carried out. And the advantages, problems and future development directions of SET index were discussed. A more reasonable understanding and application of the SET could support better thermal environment construction and prediction.