Social adjustment has been the core issue of developmental psychology. Positive adjustment is typically defined as the behavior that is consistent with social norms and benefits the individual’s survival and development across multiple domains that are self adjustment (self-affirmation), interpersonal adjustment (pro-social tendency), behavioral adjustment (acting efficiency) and environmental adjustment (active coping) respectively. Positive psychology indicates that positive adjustment is independent of negative adjustment, and the focus on positive function of social adjustment can facilitate social stability as well as the harmony between individual and environment. However, most previous research has focused on the negative function of social adjustment, such as problem behavior, negative emotion and so on. Much less is known about the developmental trajectories of positive adjustment and the contributing factors and mechanisms that underlie the development, particularly in life stage with an accumulation of changes and challenges. The transition to adolescence has been characterized as a critical period with the dramatic change resulted from school transition and sharp physical and psychological development. Thus, the development of positive adjustment plays an important role in adolescents’ healthy growth. Accordingly, a three-year longitudinal study was conducted to examine the change trajectories of positive adjustment, and how external factor (school transition) and internal factor (perceived pressure) shape the development for middle school students. A total of 369 seventh-grade adolescents were recruited from two middle schools and participated in the study. Among them, 290 adolescents (140 females and 150 males, mean age is 12.32 years, SD = 0.52) participated in all three consecutive years, in which 181 adolescents made the transition from primary school to different middle school and 108 continued in the same school (1 adolescent didn’t respond to the item). The study adopted Adolescents’ Social Adjustment Assessment Scale and Perceived Pressure Questionnaire for Middle School Students to assess four positive adjustment indices (self-affirmation, pro-social tendency, acting efficiency and active coping) and perceived pressure for four time points in November of 2012, May of 2013, December of 2013, and December of 2014 respectively. Latent growth modeling was used to examine: (1) initial levels and changes of positive adjustment in different domains over time; and (2) the prediction of both initial levels and growths in positive adjustment from school transition (time-invariant covariate) and perceived pressure (time-varying covariate). Results shows that, from grade 7 to grade 9, adolescents’ self-affirmation and acting efficiency increased in non-linear trajectory, and the rates of change decreased; pro-social tendency increased in linear trajectory; active coping decreased in linear trajectory. Meanwhile, there was little negative effect of the school transition on the change of positive adjustment. Finally, perceived pressure at given testing point reduce adolescents’ positive adjustment at that time. Our findings suggest that there are different developmental trajectories of positive adjustment in different domains. And all these trajectories are influenced by perceived pressure at different developmental stages. A better understanding of the causes and consequences of change in positive adjustment across the critical age period of adolescence can help to ameliorate intervention programs designed to assist maladaptive adolescents.