Abstract: The outdoor combustion of biomass is an important pollution source that affects ambient air quality. As important agricultural production area in China, the analysis of biomass outdoor combustion of biomass in the three northeastern provinces can provide a basis for the comprehensive utilization of local straw resources and the improvement of environmental quality. Based on the Fire Inventory from NCAR (FINN), the spatiotemporal distribution characteristics of biomass outdoor combustion fire points in three northeastern provinces from 2016 to 2020 was analyzed. Based on air quality monitoring data, the causes of heavy pollution weather were analyzed, and a grid based inventory of biomass outdoor combustion sources for numerical simulation was established. The results show that: (1) The number of fire points in the three northeastern provinces significantly decreased from 2018 to 2019 compared with 2016 to 2017. In 2020, there was an increase in fires, mainly occurring in spring and autumn, with relatively more fires in spring. In spring, there were relatively more fires mainly distributed in the Northeast Plain, namely the eastern and western parts of Heilongjiang Province and the western part of Jilin Province. The number of fire points in other areas were relatively small, showing a scattered distribution characteristic. (2) This study established a grid based inventory of biomass outdoor combustion sources in the three northeastern provinces from 2016 to 2020, with a spatial resolution of 3 km. The types of pollutants monitored include SO₂, NO_x, CO, NMVOC, NH₃, PM₁₀, PM_2.5, BC and OC. The total pollutant emissions from 2016 to 2020 showed an overall downward trend, and the total emissions of NO_x, SO₂, PM_2.5 and PM₁₀ were 3,479.81, 13,418.5, 215,702.87 and 235,495.21 tons in 2020, respectively. (3) By time fitting the outdoor biomass combustion pollutant emission data in 2019 and 2020 with air quality data, it was found that some cities experienced heavy pollution weather events during the spring straw concentrated combustion period in those two years, and the peak concentration of atmospheric pollutants was consistent with the peak emission of pollutants from outdoor biomass combustion. The results indicate that in recent years, the outdoor biomass combustion control measures in the three northeastern provinces have achieved significant results, and the overall emission of air pollutants is showing a downward trend. However, it is still one of the important factors for heavy pollution weather events in some cities in spring. For cities with high emissions of pollutants from outdoor biomass combustion, strict measures such as banning straw combustion should be further implemented. The grid based inventory of biomass outdoor combustion established in this study can better serve regional air quality numerical simulation research, apply in various aspects such as air pollution cause analysis, and provide technical support for formulating more accurate air pollution prevention and control measures.