<p>A polylactide (PLA) blend with simultaneous enhancement of strength, toughness, and heat resistance was successfully achieved by adding biodegradable poly(propylene carbonate) (PPC) and uniaxial pre-stretching. The effects of the PPC content (0 wt%–50 wt%) on the phase morphology and performance of the blends before and after pre-stretching were systematically investigated. Blending PPC initially reduced the strength, modulus, and heat resistance, but pre-stretching significantly enhanced these properties. In blends containing ≤30 wt% PPC, where PPC formed a well-dispersed island-like phase within the PLA matrix, pre-stretching simultaneously enhanced strength, toughness, and heat resistance. The optimized pre-stretched 70/30 PLA/PPC (ps-70/30) blend achieved exceptional performance: tensile strength increased from 66.9 MPa to 84.5 MPa, elongation at break dramatically improved from 6.8% to 115.1%, impact strength reached 55.1 kJ/m² (far exceeding neat PLA’s 3.5 kJ/m²), and Vicat softening temperature (VST) increased by 60.6% to 101.8 °C. Notably, the ps-70/30 blend retained excellent mechanical properties even after six months of aging. These improvements were attributed to the synergistic effects of the PPC incorporation and pre-stretching. PPC not only promoted the high orientation of the PLA molecular chains but also facilitated the formation of a stable crystalline phase during pre-stretching, thereby enhancing both the mechanical properties and heat resistance. However, when the PPC content exceeded 30 wt%, phase inversion occurred, resulting in a continuous amorphous PPC phase that degraded the overall performance. This study demonstrated that a combination of controlled PPC incorporation and pre-stretching can effectively overcome PLA’s brittleness of PLA while improving its heat resistance, offering a promising strategy for developing high-performance, fully biodegradable PLA materials suitable for industrial applications.</p>