With the realisation of motivation being an essential factor that determines the successful acquisition of English as a foreign language (EFL) in China, there has been a proliferation of studies investigating Chinese secondary school students' English learning motivation. However, empirical studies addressing motivation differences are scarce. Employing self-determination theory, this study examines the motivation differences of higher-achieving, average-achieving and lower-achieving junior secondary students in China. The Language Learning Orientation Scale questionnaire was adapted and administered to 773 students, and follow-up semi-structured interviews were conducted with 12 students to gain an enriched understanding of their English learning motivation and motivation differences. The findings revealed that there were both notable differences and similarities in motivation among the three-group students. The low-level students demonstrated the highest level of amotivation and were least intrinsically motivated to learn English. Different from the commonly held belief that higher-achieving students might be least extrinsically motivated, this study indicates that all students showed a similar level of extrinsic motivation. This paper suggests that it is crucial for English teachers to adopt various strategies to foster students' intrinsic and extrinsic motivation. Furthermore, teachers are advised to pay special heed to the motivation cultivation of lower-level English language learners.