In China, people are more concerned now than in the past about adolescents’ excessive use of smartphones. This study conceptually distinguished two types of loneliness (i.e., social and non-social loneliness) and explored their different effects on adolescents’ problematic smartphone use. The mediation mechanism of smartphone usage motivations (i.e., interpersonal connectedness and habitual escape) and underlying grade difference within these effects were also examined. Data were collected from 487 students in Beijing’s secondary schools. A two-wave design with a time lag of 6 months was used. Results indicated that both social and non-social loneliness (Time 1) positively predicted problematic smartphone use (Time 2, 6 months after Time 1), and the habitual escape motivation (Time 2) played a significant mediation role in these two effects. Further, the grade showed a marginal moderation role in the relation between non-social loneliness and habitual escape motivation. Specifically, this relationship was marginally higher in senior high school students than junior students. Implications, limitations, and future directions were discussed.