Plastic film mulching is an important agricultural practice, but its accumulation in soil threatens soil health. Here, we explored the effects of plastic film on soil carbon cycling through microcosmic incubation and estimated the excess carbon dioxide (CO2) emissions of farmland soils through Meta-analysis. The results showed that plastic film released dissolved organic carbon (DOC) into the soil, which was rapidly used by microorganisms and accelerated the decomposition of soil organic carbon (SOC), increasing CO2 emissions. Soil moisture content and soil carbon content affected the rate of DOC release from plastic. We estimated that residual plastic film farmland in six northwest provinces of China, 1.39 × 105 tons of excess CO2 emissions in 2020. The soil excess CO2 emissions was influenced by polyethylene (PE) plastic concentration and soil type. Replace PE plastic film with biodegradable film, 1.05 × 106 tons of excess CO2 emissions, 7.5 times that of PE plastics.