This quasi-experimental study compared the effects of Computer-assisted Hybrid Dynamic Assessment (HDA) and Interventionist Dynamic Assessment (IDA) on the development of organizational writing skills in L2, aiming to improve both assessment and instruction within Chinese EFL context. A total of 85 first-year university students in China participated in the study, divided into two groups: HDA (n = 42) and IDA (n = 43). The Mediational Moves, representing DA scaffolded support (prompts), were implemented through both computer-assisted and instructor-mediated interventions, facilitating DA implementation in large classes. The HDA group received a combination of computer-assisted and instructor-mediated prompts, while the IDA group was solely exposed to computer-assisted prompts. They completed three tests (pre-test, DA test, and delayed posttest) under different experimental conditions to assess macro-level organizational writing skills. Differences in Mediational Moves, Mediational Efficiency, and Learning Potential Scores (LPS) were examined, along with how Mediational Efficiency influenced the relationship between Mediational Moves and LPS. The results demonstrated that both groups improved their organizational skills, with the HDA group showing greater advancements. The HDA group exhibited more effective Mediational Moves, leading to higher LPS, and better Mediational Efficiency, emphasizing the benefits of integrating computer-assisted and instructor-mediated prompts. Mediation analysis revealed that Mediational Efficiency significantly reversed the negative direct effect of Mediational Moves on LPS, turning it into a positive effect and emphasizing its crucial role in improving learning outcomes. The results suggested that HDA offers considerable potential for improving EFL writing skills, particularly in large classroom settings.