The current study examined the effects of the three levels of green exercise on people’s psychological health using a randomized trial with a pretest and posttest design and further explored which variables of the physical environment (thermal comfort, noise, and air pollution), social environment (the number of companions and crowdedness), personality traits, physical activity (intensity and frequency), and engagement with nature may help explain experiences during the three levels of green exercise using a cross-sectional approach. Field studies were conducted to test the study’s hypotheses. The participants were 95 students from a technology university in Taiwan. The experiment comprised a 15-minute green exercise in a park. No significant differences were found in emotions and attention between the three levels of green exercise. However, a 15-minute green exercise of any level significantly improved emotions and attention. Furthermore, fatigue was significantly and negatively associated with daily transportation-related physical activity, agreeableness, and engagement with nature. Moreover, the total mood disturbance was significantly and negatively associated with engagement with nature and daily transportation-related physical activity. The degree of engagement with nature played a pivotal role in green exercise. This study provided the evidence that quantified engagement with nature is beneficial for quantified psychological health for the first time.