A significant portion of emerging adults do not achieve recommended levels of physical activity (PA). Previous studies observed associations between features of emerging adulthood and PA levels, while the potential psychological mechanisms that might explain this phenomenon are not fully understood. In this context, there is some evidence that situated decisions toward physical activity (SDPA) and exercise-intensity tolerance might influence PA level. To provide empirical support for this assumption, the current study investigated whether (i) features of emerging adulthood are linked to SDPA, which, in turn, might affect PA engagement; (ii) exercise-intensity tolerance moderate the relationship between SDPA and PA level; and (iii) SDPA is a mediator of the relationship between features of emerging adulthood and PA levels under the prerequisite that exercise-intensity tolerance moderates the link between SDPA and PA engagement. In this study a group of 1,706 Chinese college students was recruited and asked to complete a set of questionnaires assessing their SDPA, PA levels, exercise-intensity tolerance, and features associated with emerging adulthood, namely Self-exploration, Instability, and Possibility. Our results indicated that SDPA positively predicted PA levels and this relationship became stronger when exercise-intensity tolerance was used as a moderator. Furthermore, it was observed that individuals with a higher level of Instability and a lower level of Possibility during emerging adulthood exhibited a lower level of SDPA. Taken together, the results of our study provide further insights on a potential psychological mechanism linking features of emerging adulthood and physical activity.