With the development and widespread application of generative artificial intelligence (GenAI), foreign language education is facing unprecedented challenges and opportunities. How teachers of English as a foreign language (EFL) can utilize GenAI in professional development has become an important research topic that urgently needs exploration. Drawing on multiple sources of data, this qualitative study systematically explores how three middle-school English teachers, in a lesson study project, integrate and utilize GenAI in their EFL teaching and professional development. The findings identify three routes of professional development for the teachers: 1) from “technostress” to “integration and innovation”; 2) from “improvisation” to “evidence-based improvement”; and 3) from “externally driven” to “collective efficacy”. The study highlights that the EFL teachers’ professional development in the GenAI era  is essentially a dynamic evolution process, covering multiple aspects such as cognitive reconstruction, decision optimization, and deepened reflection. It also offers important implications for EFL teacher development.