The present study investigated the developmental patterns of Chinese EFL(AQ) learners’ oral language in terms of complexity and accuracy and looked into the dynamic interactions between them within the framework of Complex Dynamic Systems Theory (CDST). The data were analysed using dynamic analyses (moving min–max graphs, moving correlations and Monte Carlo Simulations). It was found that, firstly, at the group level, the general developmental trends of both complexity and accuracy showed improvements. Secondly, at the individual level, the developmental patterns were non-linear and dynamic with high degrees of variability, and individual language development was influenced by the initial states. Thirdly, the analyses revealed a complex interplay between complexity and accuracy, which gradually shifted from a clearly competitive relationship during the early stages to a supportive relationship in later stages. This shift in interaction shows that complexity goes hand in hand with accuracy, which corroborates the interconnectedness of subsystems as one of the major characteristics of CDST. The findings confirm the applicability of CDST approaches to L2 oral development and carry valuable implications for CDST theory development and oral language teaching.