Within the framework of English as a Lingua Franca (ELF), lexical competence is considered more crucial thannative-like grammatical accuracy. ELF learners require a flexible, expandable, and function-oriented lexicon that ensuresintelligibility, communicative efficiency, and effective intercultural interaction. However, traditional vocabulary teachingmodels, predominantly grounded in native-speaker norms, fail to capture the dynamic and adaptive nature of ELF communication.This study proposes a Lexicon Expansion Model (LEM) specifically designed for ELF learners, integratingprinciples of cognitive linguistics, usage-based theory, and pedagogical scaffolding. The model conceptualizes lexicaldevelopment as a multidimensional process involving semantic networking, pragmatic adaptability, frequency-basedexposure, and learner agency. Employing a mixed-methods research design, the study investigates the effectiveness ofthe proposed model through experimental implementation with university-level ELF learners. Quantitative data obtainedfrom pre-test and post-test results, together with qualitative evidence from learner reflections and discourse samples,demonstrate significant improvements in lexical diversity, contextual appropriateness, and communicative confidence.The findings indicate that the Lexicon Expansion Model represents a viable alternative to traditional vocabulary instructionby aligning lexical development with real-world ELF communicative demands. The study contributes both theoretically toELF pedagogy and practically to curriculum design, teacher education, and sustainable language learning