Ethylene-vinyl acetate-glycidyl methacrylate random terpolymer (EVM-GMA, VA = 60 wt%, GMA = 2.9%) was used as a polymeric plasticizer replacing the traditional dioctyl phthalate (DOP) to prepare a polyvinyl chloride (PVC) masterbatch. Polylactic acid (PLA) was applied to blend with the PVC masterbatch in melt to examine the epoxy ring-opening reactions between epoxy groups in EVM-GMA and end carboxyl groups in PLA in order to construct a special phase structure in PVC/PLA improving the mechanical properties and 3D printing performance. The formation and evolution of phase structures were revealed by aids of the unique rheological responses of multi-component composite system. The results showed that with the increase in EVM-GMA content, the phase morphology of PVC/PLA was refined from co-continuous phase structure to denser one and then evolved to sea-island structure. It was also found that the addition of EVM-GMA inhibited the crystallization of PLA and the cold crystallization as well. When PVC/EVM-GMA/PLA was incorporated in weight ratio of 45/5/50, the denser co-continuous phase structure with smaller domain imparted the composite a highest zero shear viscosity and viscous flow activation energy. This unique structure existed stably when the shear rate was less than 230 s(-1) or below 180 degrees C. Consequently, it endowed composite the highest impact strengths and smoothest appearance of 3D printed specimens. Therefore, moderate EVM-GMA is not only a reliable alternative plasticizer for PVC, but also an ideal modifier for the compatibility of PVC/PLA blends which presented an excellent performance in 3D printing.