Metonymic competence is a kind of thinking ability and even communicative competence that should be possessed by language learners． This study investigates the interaction between advanced Chinese EFL learners' metonymic competence and writing proficiency. A cohort of 40 third-year university learners of English was allocated into two instruction groups based on their writing proficiency and received the cultivation of metonymic competence. Afterward another two instruction groups were built according to the score of their metonymic competence and were re-evaluated about their writing proficiency. The findings show that: 1) Learners' metonymic competence can be raised by the cultivation; 2) Learners with higher writing proficiency did better than those with lower metonymic competence; 3) Learners with higher metonymic competence behaved better than those with lower writing proficiency. It is concluded that: 1) writing proficiency does exert some influence on the development of EFL learners' metonymic competence; 2) the development of metonymic competence can help raise learners' writing proficiency.