Abstract: The long-term dependence on coal for heating during winter in northern China has caused serious environmental pollution. In addition, the limitations and non-renewability of fossil fuels have prompted the development of new energy sources for clean heating. Biomass energy resources are a good option because they are abundant, clean, and sustainable. Crop straw is an important agricultural biomass energy resource in China because of its’ high yield, wide distribution, and variety. Extruding straw into briquette fuel can significantly improve its combustion performance. Referring to the Intergovernmental Panel on Climate Change (IPCC), the United Nations Framework Convention on Climate Change (UNFCCC), and the Clean Development Mechanism (CDM) methodology, and through literature research and field surveys, this study constructed a calculation method for greenhouse gas emission reduction in a straw briquette fuel central heating system, with the natural decomposition of straw as the baseline. The calculation method involves four parts: the system boundary, baseline emissions, project emissions, and leakage. The system boundary includes the disposal of waste straw in the absence of the system, the route of transporting waste straw to the straw molding processing plant, the briquetting machine used in the molding process, the boiler used in the central heating process, the route to transport the straw briquette to the heating point, and the place where the straw briquette is used for heating. Baseline emissions are the sum of greenhouse gas emissions from the natural decomposition of straw, emissions from alternative coal heating, and energy consumption emissions from chemical fertilizer production replaced by returning ash generated by straw to the field. Total emissions of the system are the sum of fossil fuel consumption emissions from engineering transportation activities, power consumption emissions from straw molding, and emissions from utilizing straw molding fuel combustion and heating. The system leakage rate is zero. Finally, the total calculation formula is as follows: net greenhouse gas emission reduction of straw briquette fuel central heating system = baseline emission − energy consumption emission in straw utilization process − project leakage. Using the above methods, a case study was conducted on a straw briquette central heating project in Lintao County, Gansu Province, China. The results showed that in the heating season from 2019 to 2020, the baseline emission of the Lintao straw briquette central heating project was 1610.08 t CO₂, the project emission was 104.67 t CO₂, and the net emission reduction was 1505.41 t CO₂, which is equivalent to reducing the CO₂ emission of 529.45 t of standard coal. For every 1 t of corn straw raw material consumed by the Lintao straw briquette central heating system, the CO₂ emissions are reduced by 1.17 t. Therefore, it can be concluded that the straw briquette central heating system has an obvious emission reduction effect. This study established a quantitative calculation method for greenhouse gas emission reduction for a straw-briquette-based central heating system based on the background of natural straw decomposition, which enriches the estimation methodology for greenhouse gas emission reduction evaluations. This quantitative study demonstrated that large-scale straw briquette-based central heating projects have a significant emission reduction effect.