With the rapid development of artificial intelligence, generative AI technology has provided a new path for enhancing the teaching design capabilities of English teachers. Instructional design ability encompasses task analysis, learner research, content organization, and resource innovation, and it is an important indicator for measuring teachers’ professional growth. At present, English teachers still face many challenges in goal setting, learner analysis, and technology integration. Generative artificial intelligence, with its features such as knowledge generation, semantic analysis, and multimodal expression, can provide task support, resource recommendation, and creative assistance in instructional design, promoting the transformation of instructional design from experience-driven to data-driven. Based on the theory of teachers’ professional ability development, this paper constructs an enabling mechanism for instructional design supported by generative artificial intelligence. It explores the mechanism and implementation path of AI promoting the professional development of English teachers from four dimensions: task alignment, learner modeling, teaching structuring, and multimodal content generation.