This book constitutes the thoroughly refereed post-workshop proceedings of the 21st Chinese Lexical Semantics Workshop, CLSW 2020, held in Hong Kong, China in May 2020. Due to COVID-19, the conference was held virtually. The 76 full papers included in this volume were carefully reviewed and selected from 233 submissions. They are organized in the following topical sections: Lexical semantics and general linguistics, AI, Big Data, and NLP, Cognitive Science and experimental studies.