The present study investigated how learners’ knowledge of a second language (L2) can influence the processing of their first language (L1). The study recorded the eye movements of 47 L1 Chinese participants with different L2 English proficiency levels as they read and judged the grammaticality of 240 Chinese sentences (60 anglicized ungrammatical relative clauses, 90 non-anglicized ungrammatical sentences, and 90 grammatical sentences). Participants were also given a language-background questionnaire (LEAP-Q) to assess their English proficiency. An analysis using mixed-effects modeling showed that acceptance of anglicized ungrammatical Chinese sentences was correlated with age of acquisition, L2 cultural identification, and L2 reading skill and exposure. These results suggest that the L1 is a more flexible system than previously thought, one that can be changed to some degree by the mastery of an L2.