Abstract: Magnesium phosphate cement, an emerging cementitious material, has broad application prospects due to its excellent properties including rapid hardening, early strength development, corrosion resistance and environmental friendliness. To optimize the composition and regulate the performance of Magnesium phosphate cement, Ethylene-vinyl Acetate polymer-modified magnesium phosphate composite materials (EMPC) were prepared via the incorporation of EVA polymer in this study. Using composite cementitious systems with varying EVA contents as research subjects, this study investigated the hydration mechanism and micro-structure evolution patterns through a combination of workability tests, macroscopic physical and mechanical property analyses, micro-structure characterization, hydration mechanism investigations. Additionally, the influence mechanism of EVA content on the early-age performance of the system was clarified. The results showed that: The polymer did not participate in the hydration process, but it can physically regulate the molecular and crystal structure, influencing the micro-morphology, pore structure, chemical composition and hydration behavior. The crystalline phases identified in the system included MgO, KH₂PO₄, MgKPO₄·6H₂O, Mg₃(PO₄)·5H₂O, Mg(OH)₂ and M-S-H. These phases, together with the EVA polymer film, formed a 3D composite network structure consisting of polymer films-MgO-struvite crystalline phases. When the EVA dosage was 2% and 4%, The spatial structure of the composite paste was the most integrated, and the mechanical strength attained its maximum value. Therefore, the optimal EVA dosage can be selected based on specific application scenarios to further broaden the application scope of EMPC.