A review of literature suggested that language teacher cognition had been used interchangeably with teachers' knowledge and beliefs, and examined as a static entity to map out the "what" aspect of teachers' minds. More recently, it has evolved to be thought of as being interactive, dynamic, and situated. However, the relationship between the two different conceptualisations remains obscure in the field. This article engages in a discussion on the conceptualisation and analysis of teacher cognition in relation to teachers' knowledge and beliefs from an activity theory perspective, drawing upon data collected from pedagogical reforms of introducing task-based language teaching for teaching Chinese as a second language. The study posits conceptualising teacher knowledge and beliefs as one element of the activity system, that is the teachers' psychological tools, with teacher cognition towards pedagogies as the tool-object negotiation activity, which involves more complexity and dynamics. The study adds to the understanding of language teacher cognition, and offers conceptual and analytical insights for conducting future research.