This mixed methods study investigated the change types and the underlying mechanisms of Chinese undergraduates' deep learning approach at a research-oriented university in eastern China. In Study 1, the deep learning approach of 273 freshmen was assessed using R-SPQ-2F at the beginning and end of a semester. The changes were categorized into three types: Type 1 (unchanged), Type 2 (increased), and Type 3 (decreased). Multiple logistic regression analysis was conducted to identify the influencing factors for each type. In Study 2, longitudinal qualitative interviews were conducted with 16 students to validate the findings from Study 1 and explore the formation mechanisms of their deep learning changes. The research revealed the changing trajectories of undergraduates' deep learning approach and highlighted the significant impact of family social and cultural capital, career goals, achievement goals, self-efficacy, and the external learning environment on these changes. The findings encourage universities to create conducive conditions that foster the enhancement of undergraduates' learning progress in higher education.