Abstract: This study first tested the working memory capacity of late Chinese-English language learners with high or intermediate second language proficiency.The results showed that there were individual differences in their capacity and at the same time, the working memory capacity was influenced by second language proficiency, and that the higher language proficiency, the higher working memory capacity.Based on the above findings, language proficiency and working memory capacity were manipulated and the judgment task on the subject-verb agreement was designed to investigate the influence of working memory and language proficiency on English syntactic processing as well as the potential interaction among them.It was revealed that when working memory capacity was similar, language proficiency exerted influence on the accuracy and processing speed of syntactic processing; when language proficiency was at the same level, working memory influenced the speed of syntactic processing.