Cellphone-use in class has many negative effects on students' overall performance. By using a self-developed monitoring-app to record students' cellphone-use in class, this study attempts to explore the relationships of cellphone-use and self-control on academic performance. The subjects of this study are 207 freshmen who take part in advanced mathematics courses, and the research period lasts for 16 weeks. Two-factor ANOVA showed that cellphone-use duration and self-control had an interactive influence on students' academic performance. There was no statistical significance in the influence of cellphone-use frequency and self-control on academic performance. Simple slopes analysis revealed a negative relationship between cellphone-use duration and academic performance for those who were low on self-control, whereas there was no relationship between these constructs for those who were high on self-control. The results show that self-control plays a moderating role in the relationship between cellphone-use duration and academic performance. Self-control could weaken the influence of cellphone-use on academic performance. Furthermore, this study is helpful to better understand the way of cellphone-use affecting academic performance, and suggests appropriate intervention of cellphone-use to help poor self-controlled students achieve better academic performance.