Previous studies have examined the effects of service-learning on student outcomes, but the dynamics and the mechanism of student development have received little attention. The present study aims to investigate how students construct their understanding of course content through service-learning, as well as the role of varied experiences. Eighty-four students were randomly assigned to two different conditions: the low-varied experiences condition (n = 36), in which students served the same child with autism throughout the programme, and the highly-varied experiences condition (n = 48), in which students served two children with autism successively. A total of 483 reflective journals written by students in a 6-week timeframe were analysed. The results indicated that students gained benefits from service-learning in terms of knowledge construction, and the overall change in students' knowledge construction fluctuated throughout the service-learning process. In addition, students in the highly-varied experiences condition also demonstrated some differences in knowledge construction changes, indicating that varied service experiences might interfere with students' knowledge construction at the turning point of task changing. The implications for service-learning and instruction are also discussed.