The present study investigates backward transfer (the influence of L2 knowledge on L1 performance) at the lexical level through the semantic gap between Chinese and Japanese words sharing the same orthographic presentation. The Chinese (L1) competence of 79 Taiwanese students with or without experience of Japanese (L2) learning was recruited for a form-meaning mapping task with an exclusively native language context. The task asked the participants to rate the degree of semantic appropriateness of three options corresponding to each orthographic word. There were 24 orthographic words in total and each word came with one Chinese meaning, one Japanese meaning, and one novel meaning for the semantic appropriateness rating. Compared to the students with no or minimal Japanese experience, the students with Japanese experience were found to provide higher rating scores for the Japanese meanings corresponding to the orthographic words in the exclusively Chinese context. The results indicate that the L1 lexical performance of those students who know Japanese had been affected by their L2 vocabulary acquisition, which was interpreted as strong evidence in support of the existence of the cross-language influence of an L2 on the L1.