Abstract: This study aims to investigate the evolution of PBAT-based biodegradable mulch films in different environments. The research objects were selected as the biodegradable mulch films with the raw materials of PBAT/PPC/additives (BDM1) and PBAT/PPC/wheat straws/additives (BDM2). Their mechanical and water vapor barrier properties were investigated with ultra-violet (UV) irradiation and deionized water (DW) immersion. The degradation process of biodegradable mulch films was characterized by microscopic morphologies, Fourier transform infrared spectroscopy, and X-ray diffraction. The results showed that both mechanical and water vapor barrier properties deteriorated under both UV and DW environments, while the rate of deterioration was greatly influenced by the cumulative irradiation intensity and the immersion duration. Compared with the films under 0.77 W/m2 UV irradiation, the films that were immersed in DW exhibited a relatively lower deterioration rate of the mechanical and water vapor barrier performance. The tensile strength loss of BDM in both machine direction (MD) and transverse direction (TD) exceeded 85% with 320 h (equal to 5.40 kJ) UV irradiation and 77% after immersing in DW for 165 d. Meantime, the WVP after 320 h UV irradiation was 2.64 and 3.85 times that of the original BDM1 and BDM2. While the WVP value after 165 d immersion was 2.22 and 3.48 times that of original BDM1 and BDM2, respectively. The FT-IR results confirmed that the degradation rate of methylene in the biodegradable mulch films was slightly lower than that of ester groups during UV irradiation. During DW immersion, the hydrolysis rate of ester groups in the films was first lower than that of methylene groups, and then the hydrolysis of the ester groups was promoted and faster than the degradation rate of methylene groups. At the same time, the addition of a small amount of wheat straws greatly contributed to the films with a slightly delayed regression of mechanical properties. The reason was resulted from the enhanced UV- and hydrolysis-resistance properties. The maximum intensities of the film usability increased from 2.2 to 2.3 kJ for the biodegradable mulching films with UV irradiation, compared with the national standards. While the maximum duration increased from 82.8 to 89.4 d with the water immersion. However, no outstanding effect was observed on the degradation of water vapor barrier properties with the addition of wheat straws. The XRD results indicate that there was no change in the degradation rate of crystals in PBAT with the addition of straws under UV irradiation. However, the hydrolysis of the crystals in the films was accelerated during water immersion with the addition of wheat straws. The variation of PBAT molecular structure was attributed to the deterioration of mechanical and water vapor barrier properties of biodegradable mulch films during aging under different environments. There was relatively less adverse impact of the aging cracks on the mechanical and water vapor barrier properties of BDM. Therefore, it is highly recommended to reduce the variation in the PBAT molecular structures, in order to reduce the deterioration rate in the performance of biodegradable mulch films.