Poly(butylene adipate-co-terephthalate) (PBAT) is considered as the most promising polymers for preparing biodegradable mulching films. However, PBAT films are suffered to severe weathering in use. This study aims to look into the effects of high amounts of micro-sized calcium carbonate and silane coated calcium carbonate on the weathering of PBAT films, as well as the effects of photooxidative degradation and hydrolysis on the weathering performance of neat PBAT and composite films. The effects of two different artificial weathering processes, namely only UV irradiation (dry condition) and UV irradiation + water spray condition, on the degradation rate of neat PBAT and its composite films were evaluated to distinguish the influences of photooxidative degradation and hydrolysis on the weathering of PBAT films. The results indicated that neat PBAT film had the fastest degradation rate under two different weathering conditions. After adding calcium carbonate, the degradation rate was delayed due to the shading effect of calcium carbonate. Comparing two different weathering conditions, it was found that degradation rates of PBAT in water spray conditions were higher than in dry conditions due to accelerated degradation rate via hydrolysis. However, in the water spray condition, the mechanical characteristics of the films containing silane-coated calcium carbonate degraded more slowly than in the dry condition. Although hydrolysis reaction accelerates the molecular weight reduction rate of PBAT, at the same time, silane molecules self-crosslink to form a three-dimensional network in the presence of water, thus compensating for the loss of mechanical properties of PBAT matrix caused by the decrease of molecular weight.