In light of complex system theory,the paper analyzes language dynamic features,the interaction,nonlinearity,butterfly effect and fossilization in second language acquisition(SLA)process,and probes into the complexity of SLA from many perspectives.It holds that language is dynamic,and SLA is a complex nonlinear system.SLA can be seen as adynamic process based on interaction,so it is sensitive to and dependent on initial conditions.The subsystems of SLA interact with social environments,promoting the development of learners’ interlanguage system.