The case study was conducted to examine the relationship between parental discourse strategies and the mixed-coding of language behavior of a 2-year-old Mandarin-English bilingual girl in interactions between the parents and the child. The parents were non-English native speakers and adopted the “one-person one-language” practice in the family, where the father spoke Chinese to the daughter and allowed the daughter to respond in either Chinese or English, while the mother used only English in the interaction with the daughter. Monthly recordings of the child’s naturalistic interactions with both parents over one year revealed that the father was more tolerant of the child’s mixed coding, and the mother was intolerant. The child used mixed Mandarin and English in interactions with her father frequently, but less frequently used the mixed coding with her mother. A series of correlation analyses shows that parents’ discourse strategies in response to the child’s mixed coding of language are positively and significantly correlated with the child’s overall rates of using mixed-coding and the likelihood that the mixed-coding was used in the next round of conversation. It seems the child differentiated pragmatic strategies in her interactions with respective parents. She recognized the one-person-one-language rule in her interactions with her mother and eventually appliedit in her interactions with her father. The significance of findings of the study is to contribute partial empirical support for Parental Discourse Hypothesis through a mixed methodology.