Family environment, emotion regulation and biological sensitivity have been shown to be associated with adolescents' externalizing problem behaviours. However, findings regarding respiratory sinus arrhythmia (RSA) reactivity are mixed and sometimes contradictory. This study aims to clarify the roles of RSA reactivity and anger regulation in the relationship between negative family expressiveness (NFE) and adolescents' externalizing behaviour by measuring RSA reactivity during the Parent-Adolescent Interaction Task (PAIT), designed to simulate a naturalistic negative family environment. In this study, 125 Chinese adolescents (M = 13.95 years, SD = 0.95; 48% male) completed questionnaires assessing negative family expressiveness, anger regulation and externalizing problems. Additionally, we collected electrocardiogram and respiration data during both the resting period and a 10-min PAIT. Results showed that anger regulation mediated the relationship between NFE and externalizing problem behaviours. Moreover, adolescents' RSA reactivity moderated this mediation effect, even after controlling for baseline RSA. Greater RSA suppression potentially indicated greater susceptibility, with the relationship between NFE and anger regulation being more pronounced in adolescents with greater RSA suppression compared to those with lesser RSA suppression. These findings highlight the importance of considering physiological systems, especially within the context of adverse family environments, when studying the relationships with externalizing problems.