The postoccupancy performance and operation management level of green building largely determines the overall sustainability level of green buildings. It is necessary to evaluate the degree of success of green buildings in the operation stage to ensure the implementation effect of the whole life cycle. However, the current green building evaluation system is incomplete, and the research on the degree of success evaluation of green buildings is very lacking. It is necessary to establish a scientific and effective evaluation index system to evaluate the degree of success of green buildings. This article applies the extension matter‐element theory and entropy method to evaluate the degree of success of green building projects, taking a green building project in Nanchang as an example, and seven experts were invited to score and evaluate the project's degree of success by calculating the relevance degree of each evaluation index. The results show that this green building's degree of success emerged as at level II, that is, the “generally successful” level, and each first‐level indicator, respectively, lay at level III, level II, level II, and level II. The case study proves that the evaluation method of green building determined in this article is scientific and reliable.