Conventional polylactic acid (PLA) melt plasticization and toughening processes are typically achieved at the expense of PLA strength and transparency, which is clearly detrimental to its application in areas such as smart home and food packaging. Herein, an ultraviolet (UV)-protective PLA-based composite (PP6) that simultaneously achieves high strength (63.3 MPa), high plasticity (125.3 %), and enhanced toughness (4.3 kJ/m2) by adding only 6 wt% poly(3-hydroxybutyrate-4-hydroxybutyrate) (P34HB) under the assist of 1 wt% chain extender was

prepared using melt blending technique. Benefiting from the cross-linking effect of the chain extender and the elongational ?ow during processing, the compatibility between P34HB and PLA, as well as the thermomechanical properties, heat resistance, and biodegradable properties of the composite, have been enhanced significantly. The extremely low melt enthalpy (1.9 J/g) and the low crystallinity PLA phase contribute to an appropriate transparency (78.3 % of glass in 400–1100 nm). The prepared composites display mid- and longwave UV-protective performance, which is superior to conventional industrial glasses. Through the superior

elongational rheology technology, PP6 maintains favorable overall properties even after six thermomechanical cycles. Collectively, the composite fabricated in this work is an attractive candidate for future applications such as smart windows, food packaging, agricultural flms, and biomedical applications.