Energy conservation in universities is significant to sustainable development, and students' behaviors hold great potential for green university development. However, few studies focused on the diversity and causality of energy-conservation behaviors (ECBs) among university students, as well as behavior differences in social characteristics. In this study, university students' ECBs were defined, including habitual ECBs, restricted ECBs, and interpersonal ECBs through literature reviews. A theoretical model of behavior causality was constructed based on the refined theory of planned behavior. Four hundred and thirty-nine questionnaire data verified the hypothesized causality using a structural equation model. Diversities in social characteristics on behavior paths and determinants were explained through multi-group analysis and one-way analysis of variance. Results show that: (1) ECBs are all affected by comfort preference and perceived self-efficacy, and perceived self-efficacy is the most critical facilitator; (2) the determinants and their importance of ECBs vary with the behavior types; (3) the effect of comfort preference, energy-conservation value, and perceived self-efficacy acting on habitual ECBs are completely mediated by energy-conservation intention; (4) gender, grade, and discipline have no moderating effect on behavior paths, while they have significant differences in behaviors and factors, and it emphasizes the necessity to focus on males, higher grades, and social science students. This work generated fresh insight into the causality of students’ ECBs and differences of behaviors and determinants in social characteristics, which make contributions to the development of ECB-related theories. Besides, some theoretical references for university administrators were provided to develop energy management measures.