Modern society makes people more and more isolated from each other, and people of all ages in the whole world have been gradually affected by this trend. It is generally believed that individual loneliness will increase with age. Previous studies have paid more attention to the loneliness of the elderly. However, in recent years, some studies have shown that the experience of loneliness will reach its peak at 20 years old. Loneliness is the most common issue that afflicts individuals in early adulthood, but the loneliness of individuals in early adulthood will be frequently overlooked because of the high expectations of society for young people's ability to adapt. Accordingly, it is particularly essential to focus on loneliness in early adulthood and its influence factors and psychological mechanisms. Social support refers to the stable material and spiritual support provided by individuals in various social connections centered on individuals. An extensive body of research have confirmed that social support is significantly negatively correlated with loneliness. A series of studies have further explored various possible intermediaries. Based on the theoretical framework of cognitive-emotional systems, this present study proposes the mediating role of social self-efficacy and negative emotions in social support and loneliness. Furthermore, based on the social problems of left-behind children that have become normalized in China, we also explored the impact of the staying experience on individuals in early adulthood. The present study investigates the relationship between social support and loneliness in early adulthood, using structural equation modeling, Bootstrap method, and multi-group analysis to examine the mechanism of social self-efficacy and negative emotions, and the regulatory role of left-behind experiences. A total of 556 college students were surveyed (262 males and 294 females; 351 left-behind experience students and 200 no left-behind experience students; aged from 18 to 25 years, average age 20.7±1.46), Participants completed the questionnaires of social support, loneliness, social self-efficacy, anxiety and depression. The results show that (1) social support, loneliness, social self-efficacy, anxiety and depression are significantly relevant. Social support is an important factor in the individual loneliness of early adulthood (r = -.32, p < .001). (2) Analyses of the structural equation model shows that the multiple mediation model fitting index reaches the critical value (χ2/df = 4.91, CFI = .95, TLI = .92, SRMR = .06, RMSEA = .08). (3) Social mediation has a significant mediating effect between social support and loneliness (effect size = 9.66%); negative mediation has a significant mediating effect between social support and loneliness (effect size = 28.37%); social self-efficacy and negative emotions act as chain mediators in the effect of social support on loneliness (effect size =10.26%). (4) The left-behind experience has a regulatory effect on the relationship between individual social support and loneliness in early adulthood. The social self-efficacy of the left-behind experience group significantly predicts loneliness (p < .001), and the social self-efficacy and loneliness of the no left-behind experience group the path coefficient of the sense is not significant (p =.238). These results give support to our theoretical hypothesis that social support is an important factor in the sense of loneliness in early adulthood, as well as social self-efficacy and negative emotions as mediating mechanisms between social support and loneliness, which also provides suggestions for psychological adjustment of individuals in early adulthood.