The work aims to improve the foaming performance of poly(butylene adipate-co-terephthalate) in supercritical carbon dioxide (CO2), and explore the application of foam in the field of hydrophobic and thermal insulation degradable packaging. Grains (Gr) were modified with silane coupling agent KH550, and then the effect of supercritical CO2 on the foaming properties of grains/PBAT/ polylactic acid (PLA) composites with different proportions was investigated. In addition, the effects of grains ratio before and after modification on the foaming performance were investigated through density testing, infrared spectroscopy, SEM, DSC, compressive resilience testing and thermal conductivity testing. The compatibility of the modified grains (KGr) with PBAT/PLA matrix was significantly improved after KH550 modification, and the grains could be used as structural framework to support the bubble structure of the foam. The foaming ratio of PBAT/PLA foam was reduced by 54.4% after 14 days of placement, but the volume of grains/PBAT/PLA foam was almost unchanged after 14 days of placement. The contact angle of 15%KGr-PBAT/PLA foam reached 125.4°, which was significantly higher than 114.3° of PBAT/PLA foam. The thermal insulation performance of 15%KGr-PBAT/PLA composite foam was also better than that of PBAT/PLA foam. The grains can be used as skeleton structure to support the pore of PBAT/PLA composite foam; at the same time, the grains modified by KH550 have better compatibility with PBAT/PLA matrix. The modified grains could significantly improve the foaming properties, hydrophobic properties and thermal conductivity of the grains/PBAT/PLA composite foam, so as to be applied to packaging materials.