Abstract: Outdoor combustion of biomass is an important pollution source affecting ambient air quality. As important agricultural production area in China, analysing the outdoor combustion of biomass in the three northeastern provinces can provide a basis for the comprehensive utilisation 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 was analysed for the three northeastern provinces from 2016 to 2020. Analysis of the causes of heavy pollution weather was achieved based on air quality monitoring data, and a grid based inventory of biomass outdoor combustion sources available for numerical simulation was established. The results show that: (1) The number of fire points in the three northeastern provinces from 2018 to 2019 has significantly decreased compared to 2016 to 2017.The number of fire points in the three northeastern provinces from 2018 to 2019 has significantly decreased compared to 2016 - 2017. In 2020, there was an increase, with the main fire points occurring in spring and autumn, with relatively more in spring. In spring, there were relatively more fire points mainly distributed in the Northeast Plain, namely the eastern and western parts of Heilongjiang Province, the western part of Jilin Province, etc. The number of fire points in other areas were relatively few, showing a scattered distribution characteristic. (2) A grid based inventory of biomass outdoor combustion sources in the three northeastern provinces from 2016 to 2020 was established in this study, with a spatial resolution of  3 kilometers.The types of pollutants monitored include SO₂, NO_x, CO, NMVOC, NH₃, PM₁₀, PM_2.5, BC, and OC. The overall pollutant emissions from 2016 to 2020 showed an overall downward trend, with the total emissions of NOx, SO2, PM2.5 and PM10 in 2020 being 3 479.81, 13 418.5, 215 702.87 and 235 495.21 t, respectively.(3) By time fitting the emission data of pollutants from outdoor biomass combustion 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 both years, and the peak concentration of atmospheric pollutants was consistent with the peak emission of pollutants from outdoor biomass combustion. The results shown that in recent years, the control measures for outdoor biomass combustion in the three northeastern provinces have achieved significant results, and the overall emission of atmospheric pollutants was 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 could 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.