The present study explored bilingual coactivation during natural monolingual sentence-reading comprehension. Native Chinese readers who had learned Japanese as a second language and those who had not learned it at all were tested. The results showed that unrelated Chinese word pairs that shared a common Japanese translation could parafoveally prime each other. Critically, this translation-related preview effect was modulated by the readers’ language-learning experiences. It was found only among the late Chinese–Japanese bilinguals, but not among the monolingual Chinese readers. By setting a novel step, which was testing bilingual coactivation of semantic knowledge in a natural reading scenario without an explicit presentation of L2 words, our results suggest that bilingual word processing can be automatic, unconscious and nonselective. The study reveals an L2-to-L1 influence on readers’ lexical activation during natural sentence reading in an exclusively native context.