Studies suggest that pre-service teachers' existing beliefs often serve as a lens to interpret the subject matter to be taught, guide their thoughts in lesson planning and shape their behaviour in classroom practices during their teaching practicum. This study examines how these beliefs affect pre-service teachers' conceptualization of Putonghua-medium-of-instruction (PMI) and their teaching. The findings indicate that the conceptualization of pre-service teachers about PMI ranged from code switching to curriculum integration. Their focus of lesson planning and teaching practice varied from transmission of Putonghua knowledge and detailed explanation of text information to integration of Chinese Language and Putonghua.