With the increasing pace of urbanization in China, millions of peasants migrate to cities to seek employment and leave their children behind in the rural area, who are called left-behind children. Presently, over 61 million left-behind children live in China, meaning that one in five child experience separation form parental migration. Without the help of migrating parents in chronic lives, left-behind children may encounter various difficulties and have considerable social and emotional costs, such as perceiving more life stress, enjoyment of less general well-being (GWB) and suffering from more loneliness than counterparts whose parents are available. In actual lives, many left-behind-children have little influenced by their parents, migration and grow up with full of positive strength (i.e. psychological capital). This study is to investigate the effect of psychological capital on the relationship between perceived life stress on loneliness and GWB among the left-behind rural children. Data used in this study was from a survey recently conducted in 98 rural villages of China’s Hunan Province. A snowball sampling was used to recruit subjects in grades 4 to 9 rural students receiving compulsory education. Once permission was granted by the subjects, caregivers to participate in this research, the research team approached the rural children and adolescents. After a qualified subject filled in a self-reported questionnaire in their own family, he or she was asked to introduce other qualified respondents to be potential participants. A total of 971 rural children and adolescents from two-parent family completed the survey, including 702 left-behind rural children (330 boys and 372 girls) and 269 non-left-behind rural children (113 boys and 156 girls). Of the total sample, 500 cases (51.5%) were from primary school and 471 (49.5%) subjects were from junior high school. The mean age of the participants was 12.00 (SD = 1.73; Range: 8～17 years). Our cross-sectional study shows that: (1) Compared to non-left-behind children, the left-behind-children report higher level of life stress and loneliness and lower level of psychological capital and general well-being. (2) After controlling for the demographic variables of gender, grade and family socioeconomic status, life stress is positively correlated to loneliness and negatively associated to GWB in left-behind-children and non-left-behind children. (3) The effect of life stress on loneliness and GWB is partially mediated by psychological capital for both left-behind-children and their counterparts, with the mediation effect being from 44.7% to 54.1%. (4) Psychological capital moderates the impacts of perceived life stress on loneliness and GWB only for left-behind-children, which prove that with the level of psychological capital increasing, the adverse effect of life stress on GWB and loneliness decrease. In conclusion, psychological capital not only mediates but also moderates the impacts of life stress on loneliness and GWB for left-behind-children. These findings suggest that the theoretical perspectives of stress-process model, conservation of resources theory (COR) and stress-buffering hypothesis can be supported in our sample. Furthermore, the results implicate that psychological capital is an important psychological resource for left-behind children to cope with life stress, and intervention aimed to improve their psychological capital can promote their mental health.