Concrete is prone to sudden brittle fracture under bending due to its high brittleness. In order to improve the flexural performance of concrete, this paper prepares two structural forms of polylactic acid (PLA) frame concrete composite materials with negative Poisson's ratio and imitation pearl layer through 3D printing. Three point bending experiments are conducted, and the flexural performance and failure mechanism of PLA frame concrete are analyzed by combining digital image correlation (DIC) technology. The results indicate that the negative Poisson's ratio framework improves flexural strength by 133.4% while reducing density by 11.26%, achieved through the auxetic effect and multi-crack dispersion mechanism. The nacre-like framework enhances flexural strength by 164.6% and reduces density by 15.06%, attributed to its hierarchical interface deflection and crack bridging mechanisms. Both frameworks significantly delay crack propagation, achieving a transition from brittle to ductile behavior. This research provides experimental and theoretical foundations for developing lightweight, high-toughness concrete structures.