This study was designed to develop and validate a game-based assessment tool for the fine-grained assessment of digital literacy in primary school students. Grounded in the Evidence-Centered Game Design (ECGD) framework, we developed a game-based assessment tool titled “Escape from the Island”. We collected data from 145 fifth-grade students and utilized Cognitive Diagnostic Models (CDM) to confirm the tool’s reliability and validity, and to analyze student mastery of literacy attributes. The results confirmed the tool’s excellent psychometric properties, including a strong model fit, a high mean reliability of 0.9883, and item guessing/slipping rates consistently below the 0.4 threshold, which together affirm its scientific validity and effectiveness. Furthermore, the assessment results revealed an imbalance in student competencies, with higher proficiency in the dimension of “digital collaboration and communication” but notable deficiencies in foundational areas like “digital knowledge and skills”. Overall, this research delivers a validated diagnostic tool that surpasses traditional evaluation methods, providing empirical evidence for personalized teaching interventions and curriculum development, while offering a practical model for future educational assessments.