Abstract: In order to solve the problem of microstructure change of HR3C heat-resistant steel used in thermal power units during high temperature service, metallographic examination, scanning electron microscopy, energy spectrum analysis and mechanical properties test were carried out on HR3C heat-resistant steel tubes of high temperature superheater which were put into operation at the same time and not put into operation in the same batch. The relationship between grain boundary width, precipitation and distribution of intragranular second phase and mechanical properties was preliminarily established. According to the research results, the aging degree of HR3C heat-resistant steel pipe was divided into four states: aging (initial state), mild aging, moderate aging and severe aging. The results could provide reference for the evaluation and classification of the aging degree of HR3C heat-resistant steel structure.