From the perspective of Vygotsky's Sociocultural Theory and Zone of Proximal Development (ZPD), dynamic assessment (DA) highlights the role of mediation in the learning process by unveiling the distance between actual learning proficiency and potential learning development. DA is more helpful in fostering students' ability to solve problems independently and in facilitating future tutoring. However, due to its large consumption in time, a vast majority of studies on DA target at a limited scale. To remove this limitation from the implementation of DA, this paper attempts to apply interventionist DA with the support of online scoring systems in EFL writing. Targeting at 44 Chinese L2 English learners, this experimental research mainly adopts quantitative methods with qualitative methods as a supplement to explore their improvement in linguistic accuracy in English writing. It is found that both non-dynamic assessment (NDA) and DA helped participants to improve their accuracy, but only in the DA group observed a remarkable statistical significance. That is, DA is more effective in promoting learning outcomes. Besides, ZPD is predictive for future learning achievement.