Based on a large learner corpus, this study aims to make a comparison between lexical measures in applied linguistics and those in quantitative linguistics to investigate the effects of task complexity on linguistic complexity in Chinese English-as-a-foreign-language learners’ oral texts. The results show that measures in applied linguistics reflect task complexity effects on learners’ oral performance only in the dimensions of lexical density and lexical sophistication, while quantitative metrics can measure task complexity effects in all three dimensions of lexical variation, lexical density and lexical sophistication, and capture the nuanced changes in tasks with varying degrees of complexity. Compared with measures in applied linguistics, the changes of quantitative measures display non-linear tendencies, which embodies the complex and dynamic changes of human language. The study demonstrates that the data-driven quantitative linguistic method can extend the research on core issues in second language acquisition.