Grounded in social–ecological system theory, the present study tested the mediating effects of maternal psychological flexibility and mother–adolescent attachment on the relationship between maternal adult attachment and adolescent anxiety as well as the moderating effects of teacher support and peer support on the relationship between mother–adolescent attachment and adolescent anxiety. In total, 1139 Chinese mothers and adolescents completed a set of questionnaires, including the Experiences in Close Relationships Scale, Parental Psychological Flexibility Questionnaire, Inventory of Parent and Peer Attachment, Trait Anxiety Inventory, and Multidimensional Scale of Perceived Social Support. The results revealed that maternal adult attachment had a positive impact on adolescent anxiety. The relationship between maternal adult attachment and adolescent anxiety was chain mediated by maternal psychological flexibility and mother–adolescent attachment. In addition, teacher support and peer support had moderating effects on the relationship between mother–adolescent attachment and adolescent anxiety. These findings support the systematic social ecosystem perspective and highlight the differences in the effects of different maternal adult attachment styles, teacher support, and peer support on adolescent anxiety.