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