This study adopts word association tests to investigate diachronic changes in organizational patterns and network structures of Chinese EFL learners’ word association networks during two decades. Results show that: 1)Chinese EFL learners’ word association networks are dominated by semantic relations, with low and high proficiency participants exhibiting different developmental trends in terms of collocational and hierarchical knowledge; learners in current stage perform a dramatic increase of morphological relations in non-semantic responses; 2)Chinese EFL learners’ word association networks possess the “small-world” property, with sparsity increasing as L2 proficiency grows and stability declining as “small-worldness” increases. This study provides a diachronic depiction of vocabulary learning and insights for vocabulary assessment.