Research impact is an important manifestation of research competence and the focus of medical education. This study is dedicated to exploring the relationships between coauthorship networks and the research impact of Chinese medical doctoral students from a social capital perspective. A total of 16291 scientific papers from 237 doctoral students and 126 mentors at Chinese universities were selected from databases, and a study dataset including 19 variables was constructed. Nine independent variables were defined and obtained through coauthorship network analysis, and the doctoral students’ research impact, as the only dependent variable, was used to test the hypothesized relationships among the variables. The results show that the betweenness centrality, student-mentor coauthorship count and the partnership ability index significantly affect the h-index. Specifically, the coauthorship unit count plays the most important role in developing centrality, which, in turn, produces a higher h-index. In addition, betweenness centrality, student–mentor coauthorship count and the partnership ability index are good predictors of the likelihood of doctoral students entering the greater research impact group and especially improving their betweenness centrality. Specifically, doctoral students whose partnership ability index is greater than 2, student-mentor coauthorship count is greater than 4, coauthorship unit count is greater than 6, and betweenness centrality is greater than 0.02 are considered to have greater research impact. These findings suggest that the important roles of cooperation in the development of research competence and good mentorship in the acquisition of social capital by doctoral students should be emphasized. Several strategies are advised for harnessing social capital rooted in doctoral students’ coauthorship network for relevant organizations, mentors and doctoral students who want to increase medical doctoral students’ research impact.