Abstract: Previous studies on doctoral students' research activities paid little attention to their research processes and the internal state of research activities, thus forming a black-box in understanding their research. Through multivariate linear regression and mediation analysis, this study has found that doctoral students' basic information upon entrance, their individual psychology and social environment, which are variables closely related to doctoral education, have a significant impact on their time investment in research, and with their time investment in research as the mediator, these variable have an impact on their research productivity, while the impact of their time investment on their research productivity is relative weak. This indicates that the time investment in research cannot be used as the core mediating variable to analyze doctoral students' research process. In addition, both the quality and quantity of their research time should be taken into account to expand and enrich the connotations and dimensions of their research engagement.