Students enrolled in secondary schools often fail to engage in moderate to vigorous levels of physical activity (MVLPA) due to inadequacies in their physical education programs. Physical education teachers (PETs) foster student involvement in MVLPA. Consequently, it becomes imperative to identify and scrutinize factors pertaining to PETs that could influence students’ MVLPA within secondary school settings, an area that has been relatively overlooked in China. To address this gap, the present study delved into MVLPA among secondary school students in southwest China with the following objectives: (i) to assess students’ MVLPA alongside PET characteristics and teaching behaviors during physical education classes; (ii) to explore discrepancies in PET behaviors and characteristics; and (iii) to establish connections between students’ MVLPA and PET behaviors and characteristics during physical education sessions. A questionnaire survey was administered to 54 full-time PETs across nine secondary schools in Chengdu, China, aimed at gathering data on their personal and professional attributes. Additionally, a system for observing fitness instruction time was employed to document PET instructional traits, while accelerometers were utilized to track students’ MVLPA. The study objectives were investigated using multiple statistical analyses. The findings indicate that PETs do not meet the recommended 50% MVLPA time allocation during physical education sessions. Noteworthy patterns emerged, revealing that PETs with 1–5 years of teaching experience allocated less time to student observation and more time to classroom management compared to their counterparts with 6–10 years and over 10 years of teaching experience. Moreover, MVLPA time invested in lessons led by male PETs (B = −3.221) was significantly higher than time spent in lessons led by female PETs, which was attributed to PET gender. Furthermore, students under the tutelage of PETs with 6–10 years of teaching experience (B = 3.101) and those with over 10 years of experience (B = 2.989) exhibited significantly higher MVLPAs than under those with 1–5 years of teaching experience. Additionally, PET attitudes such as observation (B = 1.621) and promoting (B = 1.317) behaviors during physical education sessions were positively correlated with students’ MVLPA. A regression analysis revealed that PET characteristics and behaviors explained 21.3% of students’ MVLPA variance. This study offers insights into PETs’ pivotal role in promoting physical activity. It underscores the ramifications for students’ MVLPA in the Chinese educational context.