This study explored the role of vocabulary breadth and depth in second language learners' performance in IELTS academic reading tests in China. Sixty-two Chinese learners of English as a foreign language completed a vocabulary size test, a vocabulary depth test, and an IELTS reading test. Results showed that vocabulary breadth and depth both correlated significantly with IELTS reading test scores. With regard to different IELTS question types, vocabulary breadth correlated significantly with True/False/Not Given questions, whereas vocabulary depth correlated more significantly with Multiple Choice, Matching Headings, and Sentence Completion tasks. Results of a multiple regression model indicated the increase of vocabulary size needed to improve certain IELTS band scores. This study has theoretical implications for broadening the conceptualisation of vocabulary depth, as well as pedagogical implications for supporting students' second language reading development.