Objective To explore the relationship between patients′ participation competence and perceived shared decision making among cancer patients, and to provide reference for promoting patient involvement in treatment decision-making. Methods A cross-sectional study was conducted involving a convenience sample of 458 cancer patients from 4 public hospitals in Guangzhou. Patients′ self-reported data were collected using the Patients Participation Competence Scale (PPCS) and the 9-item Shared Decision Making Questionnaire (SDM-Q-9). Hierarchical multiple regression analyses, with a control of patient sociodemographic and clinical variables, were conducted to explore the effect of participation competence on perceived shared decision making. Results The median total score of perceived shared decision making was 65.67(IQR:44.44,79.00) points,and the total score of patient participation competence averaged (96.80 ±21.02) points. After controlling the sociodemographic and clinical characteristics, the domains "information seeking ability", "communication ability", and "autonomous decisions ability" of PPCS still accounted for 37.5% of the variance in perceived shared decision making (P<0.05 for all). Conclusion Health professionals should pay attention to the evaluation of patients′ participation competence, and take measures to improve patients′ information seeking ability, communication ability and autonomous decisions ability, so as to promote shared decision making in clinical practices.