Biodegradable long-chain branched (LCB) poly(butylene succinate-co-butylene terephthalate) (PBST) copolyesters were prepared via a two-step direct esterification and melt polycondensation process, using a small amount (0-2 mol 96) of diglycidyl 1,2,3,6-tetrahydrophthalate (DGT) as an in situ branching agent (BA). The chemical structures of LCB PBSTs were characterized and the thermal, mechanical, and theological properties were investigated. With increasing DGT loading, PBSTs with higher branching degree, broader molecular weight distribution, and lower melt flow index were synthesized in shorter polycondensation time. The branching of PBSTs results in a slight decrease in crystallizability, melting, and Vicat softening temperatures, but leads to an obvious decrease in elongation at break. On the other hand, the existence of LCB greatly improved the rheological properties of PBSTs. PBST with higher branching degree possesses higher storage and loss modulus, higher zero-shear viscosity, longer relaxation time, more obvious shear-thinning, and lower loss angle tangent. The Han plot of the rheological data also indicates higher elasticity of LCB PBSTs.