The use of massive open online courses (MOOCs) for teacher professional development (TDP) has increased in the past decades. This study explored the key factors that influenced teachers' online course completion as a significant indicator of their success in a TPD MOOC. Six key influencing factors (self-efficacy, interaction with curriculum content and peers, satisfaction, overall course score, and TPACK self-assessment) were identified from the perspectives of general influencing factors and teacher specific influencing factors. Employing a learning model that was constructed as a hypothesis path model based on literature review, we analysed these influencing factors' effects on participants' online learning completion. Results showed that participants' TPACK self-assessment and overall course score had a strong direct effect on their course completion, while their interaction with course content and peers had a significant indirect effect on their course completion. This study also discussed the features of teachers' learning in TPD MOOCs based on the results. Suggestions to promote the success of TPD MOOCs from the aspects of curriculum, learners, and community were put forward. Implications and prospects for research and practice are discussed.