According to the second language acquisition research framed in the Dynamic Systems Theory, learning motivation is in constant and complex changes. Thus demotivation, the opposite of motivation, experiences corresponding fluctuations. The mainstream demotivation research prefers cross-sectional investigation into factors leading to learners’ demotivation, but not in a dynamic perspective and even ignores their individual differences. Therefore, the current study, from a longitudinal perspective, adopts a qualitative method of semi-structured interviews to examine the dynamics of demotivation in an undergraduate interpreting classroom and factors underlying these changes. It is found that the demotivation dynamics of students have different characteristics which are difficult to be generalized by group averages. Reasons include lack of learning interests and goals, difficulty in learning and a fast pace in teaching. Besides, the same teaching contents will evoke opposite changes in students’ motivation. Some students’ motivation increased while others’ motivation decreased. By exploring the individual differences and dynamic changes of learners’ demotivation, English teachers are able to monitor students’ learning in a more comprehensive way and to provide tailored instruction. This study can also supplement the research on demotivation by a dynamic approach.