This paper presents a qualitative study to investigate the dynamics in second language(L2) learning strategies under theguidance of the complexity theory. A group of Chinese undergraduate students studying at an international university in Thailandwere selected as the research participants. Research instruments include interviews, observations, records of participants' on-linechat and posts, and a research journal. The research findings indicate that the changes in the participants' strategies for learningEnglish exhibit typical features of the complex system. The study will provide implications for probing into the nature of L2 strategyand for applying complexity theory to future researches on L2 strategies.