Heavy haze events occur frequently over Northeast China during winter, despite the successful implementation of the Clean Air Act, which primarily targets fossil fuel sources, in recent years. Agricultural fires have been suggested as one of the main causes of these haze episodes. However, their regional contribution to fine particulate matter (PM_2.5) pollution has not been systematically evaluated. In this study, we use the GEOS-Chem model to investigate the role of agricultural fires in heavy haze episodes from December 2018 to March 2019 in Heilongjiang Province. Our results show significant discrepancies between the simulated and observed PM_2.5 concentrations during severe haze pollution days. By increasing agricultural fire emissions in the GFED4s inventory by approximately a factor of 23, we are able to better replicate the haze episodes in the model, indicating a severe under-representation of agricultural fires in the inventory. Furthermore, the baseline simulation overestimates the observed black carbon (BC) to organic carbon (OC) ratio in Harbin, suggesting a biased emission ratio specified in the inventory. Our assessment underscores that agriculture fires constitute the main cause of the extreme haze episodes during the study period and a strictly implemented ban would improve air quality with substantial health benefits.