With the popularity of smartphones, their impact on the physical and mental health of left-behind adolescents has become the focus of social attention. Some believe that smartphone use will provide emotional compensation for left-behind adolescents, while others believe that the lack of supervision of smartphone use among left-behind adolescents will exacerbate their psychological problems. To explore the mechanisms of the dual effects, we conducted a field survey of 848 left-behind adolescents (male = 446, age between 12 and 19) in Shicheng County in Gannan Region of China. It was found that social smartphone use, recreational smartphone use, frequency, and duration of smartphone use, mediated by smartphone dependence, were positively associated with social anxiety, stress perception, and sleep disorder, and further with depression of left-behind adolescents. Although social uses of smartphones were positively associated with online social support, they were not related to loneliness and depression. Based on our results, left-behind adolescents' smartphone use habits have shown more harmful than good effects on their psychological well-being.