Loneliness is a critical factor that can significantly affect an individual's health and well-being, especially for older adults. Since they are more vulnerable to suffering life transitions, older people are more likely to experience loneliness than people of younger ages. The fast break out and spread of COVID-19 exacerbated the difficulty of their social connections and intensified their feelings of loneliness. Consequently, effective interventions for the aged, particularly effective ones suited to special occasions, are of vital relevance. Consequently, effective interventions for the aged, particularly effective ones suited to special occasions, are of vital relevance. As one of the widely mentioned and applied gerontology theories, the activity theory has been well-studied previously to elucidate the effectiveness of leisure activity participation in reducing lonely feelings. It is worth noting that no one solution works for everyone. This thesis examines the alleviating roles of four types of social activities on loneliness in older Americans considering individual differences, including marital, job, and physical health background. Sample data is from the 2020 Health and Retirement Study (HRS) survey. This thesis collected respondents over 50 years old and included a sample of 4,506. According to previous studies, control variables include age, gender, race and ethnicity, years of education, and health components (self-rated health and memory, COVID infection, and depressive symptoms). The thesis divides social activities into highly productive-active, moderately productive-active, moderately productive-sedentary and consumptive activity. Through three-group regression analysis, this thesis concludes that: (i) highly productive-active activity was the most practical way to reduce loneliness for vulnerable groups; (ii) moderately productive-active activity had the best utility on the elderly who already maintained good social relationships and health status; (iii) moderately productive-sedentary was the only activity type not useful for all groups. (iv) consumptive activity presents a protective tendency towards older people disadvantaged in work but not towards married and partnered people. Meanwhile, the thesis also proposes other classification methods involving activities' cultural and symbolic meanings. The thesis states that activities with spiritual power could better alleviate the loneliness of vulnerable groups, and the symbolic meaning of daily life activity (routine activity) could protect the elderly from isolated feelings caused by being forced out of society to some extent. The repetitive life patterns keep the elderly from inordinate life and psychological gaps. The contribution of the thesis consists of two aspects. First, the thesis maps leisure activities into a three-dimensional system, divided by practical activity attributes for loneliness. From the process of grouping and categorizing, it is possible to provide a more practical understanding of the social connection. Second, the thesis demonstrates the necessity to consider the diversity of older people. Activities were associated differently with loneliness in groups with distinct characteristics. The thesis found that context discrepancy profoundly affects the effectiveness of interventions.