While the relationship between language experience and foreign language aptitude (FLA) has been studied extensively, little is known about the underlying intermediate link between them. This study investigates whether multilingual learning experience is correlated with FLA and whether working memory (WM) mediates this relationship. A total of 93 accomplished bilingual or trilingual learners aged 18-25 years from Chinese universities participated in this study. Results showed significant differences in the performance of implicit language aptitude (ILA) (measured by LLAMA D) between L2 and L3 learners. Critically, further mediation analysis demonstrated that WM acted as a partial mediator between the multilingualism -- ILA association. Taken together, these findings highlight the partial indirect effect of multilingual learning experience on FLA through WM, which contributes to the understanding of the underlying mechanism between language experience and FLA as well as the extent to which WM affects their relationship.