Abstract: The melt blending method was used to introduce acetyl tributyl citrate (ATBC) and triethyl citrate (TEC) separately, addressing the issues of insufficient interfacial compatibility and reduced mechanical properties of the poly(butylene succinate-co-terephthalate) (PBST)/polyhydroxyalkanoate (PHA)/lignin ternary composite system caused by the aggregation of lignin. The composite materials were characterized using Fourier-transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), water vapor transmission rate (WVTR) measurement, mechanical testing, and other methods. Results showed that no new chemical bonds were formed during the melt blending process, and the addition of citrate esters slightly decreased the thermal stability of the composite materials, but significantly improved the dispersion of lignin in the PBST/PHA matrix. Compared to TEC, ATBC exhibited better dispersion effects. When the ATBC mass fraction was 1.5%, the tensile strength of the composite material reached 13.2 MPa, and the elongation at break reached 1196.9%. The surface of the material was also smoother, and the water vapor transmission rate reached 6.8 g/(m²·d), demonstrating excellent water vapor barrier properties. In addition, the addition of the two citrate esters showed improved melt flowability for the composites. When the TEC mass fraction was 1.5%, the melt flow index (MFR) reached 20.9 g/10 min. Soil degradation experiments indicated that the degradation efficiency of the composite materials initially decreased and then increased with the increasing of citrate content, illustrating a regulatory effect of the citrate esters on the material degradation. In summary, the use of citrate esters for improving the compatibility of composite materials enhances their application prospects in green packaging and agricultural films.