Previous research has demonstrated shyness plays a critical role in the development and maintenance of smartphone addiction. The present study aimed to examine the mediating roles of maladaptive cognitive emotion regulation strategies and experiential avoidance as well as the moderating role of online social support in the relation between shyness and adolescents' smartphone addiction. A sample of 513 Chinese adolescents (M[subscript age] = 16.16 years, range from 14 to 19) was recruited to complete the questionnaires. After sex and age were controlled, shyness was significantly and positively associated with adolescents' smartphone addiction and this relation was partially mediated by both maladaptive cognitive emotion regulation strategies and experiential avoidance. The multiple mediation analysis further indicated that maladaptive cognitive emotion regulation strategies and experiential avoidance exerted a mixed mediation effect on the relation between shyness and adolescents' smartphone addiction. Moreover, online social support significantly moderated the relation between shyness and smartphone addiction and this relation is much weaker for adolescents with high online social support. These results contribute to design the effective interventions of adolescents' smartphone addiction.