Built on Vygotsky's Sociocultural Theory, Dynamic Assessment (DA) integrates teaching and assessment through mediator-learner interactions to promote learner development. This study employed interactionist DA to diagnose Chinese university EFL learners' reading difficulties and promote their reading proficiency in a seven-week study. The design included a pre-test, a four-week Enrichment Program, a post-test, and a transfer test. Five learners completed each test both in a non-dynamic (NDA) and DA form. The learners' individual interactions with a mediator in DA were recorded, transcribed and analyzed via Nvivo. In addition, the learners' independent performances (IPs) on the NDA and DA, difficulties encountered in the process, the mediator's prompts provided for the learners, and the learners' mediated performances (MPs) were all identified and analyzed. Comparisons of the learners' IPs and MPs across the tests showed that DA contributed to learners' reading proficiency development, and this progress was evident both in their post-test IPs and MPs.