This study examined changes and stability of subjective well-being by dividing the state of subjective well-being into occasion factors and trait factors from a state-trait perspective during middle school. The subjects were 1,405 + 1,185 does not equal 2,325. One of the three numbers is incorrect. from the 2018 Korean Children and Youth Panel Survey (KCYPS). The results of this study show that the TSO model, which includes trait factors and occasion factors, was more suitable than the SO model for subjective well-being in middle school students. In the TSO model, trait factors explaining subjective well-being were 30.9% to 45.7%, and occasion factors were 54.3% to 69.1%, indicating that occasion factors were higher than trait factors. From this perspective, subjective well-being in the middle school years is a construct greatly influenced by occasions rather than traits. The autoregression coefficient of occasion factors was more stable as the grade level increased. The significance of this study is that it sought ways to increase subjective well-being in the middle school years by examining changes and stability by dividing the state of subjective well-being in the middle school period into traits and occasions.