Abstract: The high sodium content of Zhundong coal will lead to the serious fouling and slagging on the heating surface, and endanger the safe and economic operation of the boiler. It is an effective method to prevent and control the problem of serious fouling and slagging by mixing Zhundong coal and reducing the concentration of alkali metal sodium in the coal ash. In this study, a 600 MW opposed boiler was simulated, the alkali metal sodium precipitation, reaction and NaCl vapor condensation process was calculated, the migration and transformation process of alkali metal sodium in large-scale boiler co-firing with Zhundong coal was simulated, and the distribution of NaCl vapor concentration in flue gas and the condensation characteristics of NaCl vapor on the heating surface with the change of co-firing ratio of Zhundong. The results show: the concentration of NaCl vapor in the flue gas and the condensation rate of NaCl vapor on the heating surface of the furnace all increase with the increase of Zhundong coal blending ratio. When the blending ratio of Zhundong coal exceeds 80%, the NaCl vapor condensation rate of the heating surface increases most in the screen area, and the adsorption tendency to fly ash particles is significantly enhanced, the slagging tendency of the heated surface increases; when the blending ratio of Zhundong coal exceeds 70%, the vapor condensation rate of NaCl on the water wall increases most, and the adsorption tendency to fly ash particles is significantly enhanced, the tendency of serious fouling and slagging on the water wall is significantly increased. Controlling the ratio of Zhundong coal within 70% is beneficial to reduce the tendency of fouling and slagging in heating surface in the furnace. The research results can provide reference information for the safe blending of Zhundong coal in coal-fired boilers.