In constructivist approaches to research on career adaptability it has been conceptualized that the development of one’s career is formed from the interplay between the individual and the environment. In this study we utilized structural equation modeling analysis of longitudinal data obtained from 145 Chinese undergraduate students to examine the effects of social support and career decision-making self-efficacy on career adaptability. Our results provided important evidence regarding the effects of career decision-making self-efficacy, which functions as a significant mediator of the effects of social support on career adaptability. We provide integrative conclusions for explaining the relationships between the environment, the individual, and the individual’s career outcome, and have enriched constructivist theories of careers, providing implications for counseling and practice.