<p>Lexical richness is a critical dimension for measuring Chinese as a second language writing quality. The existing literature chiefly discusses the synchronic characteristics of Chinese lexical richness, while paying less attention to its dynamic development and longitudinal changes. Based on Dynamic Systems Theory, this empirical research investigates the diachronic features of lexical richness and the dynamic interactions among lexical indices in writings (<i>n</i> = 150) produced by 15 Cambodian-speaking CSL learners over a school year. The results are as follows: (i) The development of CSL lexical richness is a nonlinear and fluctuating process, which fully reflects the dynamicity and complexity of lexical development. (ii) There are potential interactions among the internal indices of lexical richness. Specifically, there is a supportive interaction between lexical variation and lexical sophistication, while the lexical error rate exhibits competitive interactions with both lexical variation and lexical sophistication. (iii) Under the same learning environment, there are still obvious individual differences in the development of lexical richness. The above findings indicate that the development of lexical richness in CSL writings is a nonlinear process characterized by evident dynamicity and complexity, which requires researchers to break away from the static analytical paradigm and reexamine the entire process of Chinese lexical acquisition.</p>