In order to improve the packaging performance of sodium alginate (SA) films, carvacrol (CAR) / SA composite films were prepared using the solvent casting method. And the chemical structures, and morphologies were investigated using SEM, XRD, and FTIR. The thermal stability, physical, chemical, antifungal, and preservative activities of the composite films were also studied. These results showed that the roughness of the composite films has increased. And the packing of SA chains had been altered by the CAR due to the formation of strong interaction, namely hydrogen bonding and electrostatic, between SA and CAR. The thermal stability increased after adding CAR and then decreased obviously. The resultant composite film containing 0.8% CAR (volume fraction) exhibited the optimal comprehensive perfor-mance with a water solubility of 90.48%, a transparency of 0.23 mm-1, a water vapor transmission rate of 0.17 g/(d•cm2), a tensile strength of 72.24 MPa, an elongation at break of 83.41% and an inhibition efficiency against Trichoderma sp. of 65.79%. The CAR/SA composite films could effectively maintain the quality of postharvest white mushrooms and extend their shelf life.