Abstract: Based on the dynamic system theory, four non-English major vocational college students were selected as the research objects to explore the changing trajectory and influencing factors of demotivation in English learning. The results show that motivation is a dynamic process, and its dynamic change trajectory has individual differences. The change of demotivation is the result of the interaction between internal and external factors. The initial state of demotivation has sensitive difference. The higher the initial level of demotivation is, the wider the range of fluctuation is; There are attractor and repeller states in demotivation system. Examination is the main cause of attractor state, and repeller state often appears after examination.In order to alleviate students' demotivation, teachers should guide students to strengthen their ideal L2 self, teach scientific learning methods, and pay attention to the dynamic change of students' demotivation, so as to enhance students' interest in English learning.