By analyzing the results of a word association test and a controlled productive vocabulary test of EFL learners from three successive grades, the current research investigated the correlation between the organization of mental lexicon and the development of EFL Learners’ productive vocabulary. The results showed that: (1) The syntagmatic associations in different stages increased significantly and the productive vocabulary had shown a significant development trend between the second and third grade; (2) The productive vocabulary was positively related with paradigmatic association in the first grade and with world knowledge association in the second grade; (3) Generally speaking, the combination of different types of word association and, specifically, the syntagmatic associations had predictive effects on the development of productive vocabulary.