Lesson study (LS) is an effective means of improving teachers' facility with teaching design. This research designs a case study of online to offline (O2O) LS and explores the social interaction and behaviour patterns in the process of the LS. The participants included 29 chemistry teachers from 10 secondary schools and two educational technology experts from a university in western China, who participated in the O2O LS, with Chemistry Teaching Design based on Augmented Reality (AR): Molecules and Atoms as its topic. Through social network analysis (SNA) and lag sequence analysis (LSA), the findings of this case study indicate that: (1) in the LS process, the two co-hosts played their due roles well, while different teachers played strong guiding and controlling roles in different discussion phases, and (2) in different discussion phases, the behaviour sequence of teachers' knowledge construction presented different characteristics in that high-level knowledge construction took place among teachers in the later phases under the leadership of the host. This study will aid in the design and implementation of O2O LS and the description of interactive characteristics in teachers' collaborative learning so as to provide reference for using teacher training process data and improving teachers' professional abilities.