For Chinese EFL students, learning English has always been a great challenge, and research has shown that cognitive and affective factors increasingly become important issues in the field of foreign language learning and teaching (Naderifar and Esfandiari 2016). This study is designed to investigate the relationship between learner autonomy, ambiguity tolerance, foreign language classroom anxiety (FLCA) and English achievement. It also explores the inner mechanism of the relationship between learner autonomy and English achievement in Chinese EFL undergraduates. A sample of 291 third and fourth grade undergraduates in China was assessed for their levels of learner autonomy, ambiguity tolerance and foreign language anxiety using the Learner Autonomy Questionnaire (LAQ), Second Language Tolerance of Ambiguity Scale (SLTAS) and Foreign Language Classroom Anxiety Scale (FLCAS), respectively. Participants' scores of the College English Test Band Four (CET-4) were used to measure their achievement in English. The results revealed a link between learner autonomy, ambiguity tolerance, FLCA, and English achievement. Ambiguity tolerance and FLCA mediated the relationship between learner autonomy and English achievement, accounting for 36.58?% of the total effect. These findings suggest a process through which ambiguity tolerance can decrease FLCA and identify the mediating effects of ambiguity tolerance and FLCA in the relationship between learner autonomy and English achievement in Chinese EFL undergraduates. Pedagogical implications of the study are presented and discussed.