The application of artificial intelligence(AI)in foreign language education has been transforming perceptions and practices of language teaching and learning.However,so far little research has been done on the ways in which students perceive and use AI in foreign language learning.This study employs a qualitative thematic analysis to investigate students'perceptions and use of AI in English academic writing.The findings reveal that AI assists students'writing in two aspects:enhancing writing quality and efficiency.The two-level assistance encompasses four dimensions of optimizing language,processing literature,composing content,and providing strategies.Students have a reasonably clear understanding of the advantages and limitations of each AI assistance dimension,and thereby form typical behavioural patterns.The study further discusses potential ways of reconstructing English academic writing competencies in the era of digital intelligence,hoping to offer new insights into building an intelligent foreign language education system empowered by AI-human collaboration.