To develop novel blend materials combining the performance advantages of poly(butylene adipate-co-terephthalate) (PBAT) and polylactic acid (PLA), in this study, the block polymer PBAT-b-PLA was introduced into the PBAT/PLA blend system, forming the ternary blend PBAT/PBAT-b-PLA/PLA. The compatibilizing effect of the block copolymer on the blend and its impact on mechanical, optical, and gas barrier properties were systematically investigated. Differential scanning calorimetry and scanning electron microscopy analyses reveal that PBAT-b-PLA significantly enhances the compatibility between PBAT and PLA. The dispersed phase size and the difference of glass transition temperatures (ΔTg) between the two phases decreases from 5712 nm and 94.22 °C in the PBAT/PLA blend to less than 1200 nm and 94.00 °C, respectively. The dispersed phase size decreases as the number average molecular weight of PBAT-b-PLA is reduced; when the molecular weight decreases from 8.5?104 to 3.0?104, the dispersed phase size decreases from 1186 nm to 426 nm. The compatibilization by PBAT-b-PLA endows the ternary blend PBAT/PBAT-b-PLA/PLA with superior properties compared to PBAT/PLA, and these properties vary with the molecular weight of PBAT-b-PLA. As the molecular weight decreases from 8.5?104 to 3.0?104, the tensile strength of the ternary blend remains stable at approximately 30 MPa, while the elongation at break increases from 116.1% to 276.1% — significantly higher than the 15.6% observed in PBAT/PLA. The onset wavelength for light transmission gradually decreases, whereas PBAT/PLA exhibits no transmittance in the visible light range (400~780 nm). The water vapor permeability coefficient (P_(H_2 O)) decreases from 1.23×10-13 g·cm/(cm2·s·Pa) to 1.18×10-13 g·cm/(cm2·s·Pa), and the oxygen permeability coefficient (P_(O_2 )) decreases from 2.32×10-14 g·cm/(cm2·s·Pa)to 2.18×10-14 cm3·cm/(cm2·s·Pa), both lower than those of PBAT/PLA, indicating enhanced gas barrier performance.