<p>Various life activities, such as sleep, physical activity (PA), screen use, and learning, are closely related to the mental health and subjective well-being (SWB) of the old adults. Previous studies have focused on a single life activity and used flawed statistical methods, which lead to divergent results, as co-dependency between activities were neglected. This study aimed to explore the relationship between daily activity time and depression and SWB in the old adult group from the perspective of time composition. In this study, we collected data on the sleep, moderate-to-vigorous PA (MVPA), light PA (LPA), screen use, and learning times of 1,738 old adults from 28 provinces in China. Analyses were performed using a compositional data analysis (CoDA) and isotemporal substitution model (ISM). The study results show that, considering changes in time spent on other activities, learning and LPA were positively associated with SWB and negatively associated with depression. In contrast, MVPA time was negatively associated with SWB and positively associated with depression. Screen use was not associated with SWB but positively associated with depression, and sleep was negatively associated with SWB but had no correlation with depression. The ISM showed that the most effective way to improve SWB and reduce depression is to reduce time spent on MVPA and increase time for learning or LPA.This study was the first to analyze life activities from the perspective of time composition (how to change life activities’ time allocation) to improve well-being and reduce depression among Chinese old adults and provide some practical life advice.</p>