With increasing attention being paid to teacher education, various theoretical perspectives have been adopted to study on how pre-service teachers learn to teach. This study employed complexity theory as conceptual framework and examined two pre-service EFL teachers' practicum experiences by qualitative case study and identified three core characteristics of their learning-to-teach: complexity, dynamism and self-organization. It is found that pre-service EFL teacher learning, as a complex system, interacted between and among a network of socio-contextual factors and thus influenced and being influenced during the learning process in a non-linear and complex way. This study made empirical and theoretical contributions to the research field of teacher education and provided implications for university-school collaboration-based teacher education practice.