Among various study topics of advanced second language (L2) learners, mental lexicon shares a unique significance. This paper will introduce a comparative experiment between advanced Chinese English as a Second Language (CESL) learners and English as first language (EL1) speakers. The research question of the study is whether advanced CESL learners have a different English mental lexicon organization compared with EL1 speakers. The study adopts the word association test as its main research method. The main finding of the experiment is that advanced Chinese ESL learners' mental lexicons are less active and more limited than EL1 speakers. Based on the finding, the study concludes that the two test groups may apply different models for their mental lexicon organizations. The result of the ESL group prefers the Similarity-based Model while the mental lexicon organization of the EL1 group fits the Theory-based Model. Finally, the paper discusses the pedagogical implications of the study. It confirms the importance of lexical instruction in advanced L2 instruction and suggests combining vocabulary's semantic information with various social contexts to create effective lexical instruction for learners. (Contains 7 tables.)