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Herein we developed a novel strategy for preparing biodegradable polylactide (PLA) based materials with improved crystallinity, mechanical properties and rheological behaviour by introducing a long-chain branched block copolymer (LB-PCLA) of PLA and poly-3-caprolactone (PCL). The LB-PCLA copolymer was synthesized by single hydroxyl-terminated PLA (PLA-OH) and three hydroxyl-terminated PCL (PCL-3OH) precursors. The crystallinity and crystal morphology of PLA/LB-PCLA blends were investigated by a differential scanning calorimetry (DSC) instrument and polarized optical microscopy (POM). The morphology and domain size of PLA/LB-PCLA blends were investigated by transmission electron microscopy (TEM). The irregular dispersed droplet shape of the LB-PCLA copolymer suggested that the interfacial interaction between the PLA and PCL phases was obviously compatible because of the copolymerization and the branched structure of the LB-PCLA. This phase morphology is responsible for the enhancement in crystallinity, crystallization rate, and toughness of the PLA/LB-PCLA blends compared to neat PLA and PLA/PCL blends. The elongation at break for the PLA/LB-PCLA blend with 15 wt% of the LB-PCLA copolymer was about 210%, an increase of 30 times compared with that of neat PLA. The rheological behaviour also shows that the LB-PCLA copolymer and PLA/LB-PCLA-15 have more pronounced shear thinning behaviour and longer relaxation time than neat PLA and PLA/PCL blends with 15 wt% of the PCL, which can be attributed to the long-chain branched structure of the LBPCLA copolymer.