The treatment process of biomass waste will release a considerable amount of greenhouse gases. Under the background of "carbon peaking and carbon neutralization", effective low-carbon treatment technology should be adopted to reduce the carbon emission of the treatment process. Through anaerobic digestion, biomass waste is transformed into biogas, and then purified to obtain high-purity methane, which can replace fossil fuels and significantly reduce greenhouse gas emissions. Based on CM-072-V01 methodology, this paper calculates the carbon emission of a mixed anaerobic digestion project of sludge and kitchen waste, and calculates the baseline emission value, project activity emission value, leakage value and total carbon reduction. The calculation results show that the carbon emission reduction of the project is significant.