In order to explore the effects of mixed coal combustion on reducing the carbon content of fly ash in a subcritical 600 MW natural circulation balanced draft boiler, three types of coal with significant differences in characteristics were used, and numerical simulations were combined with experiments to study the mechanism of mixed coal combustion and its relationship with fuel characteristics, burnout rate, and carbon content in fly ash in the stratified combustion process of the balance boiler. Based on the results, a set of coal blending principles are proposed to improve the combustion characteristics of coal powder in power station boilers. Firstly, three extreme operating conditions were set up, and high-quality coal was respectively focused on the upper, middle, and lower layers of the burner for combustion. And then, through computational fluid dynamics (CFD) simulations, it was found that when the coal powder airflow of the balance combustion boiler flowed upward, and the high-quality coal was distributed in the upper layer of the combustion chamber while the poor-quality coal was distributed in the lower layer, the residence time of poor-quality coal in the furnace increased significantly, allowing it to fully combust in the high-temperature area of the upper layer, resulting in a low carbon content in fly ash. Furthermore, based on the analysis of the heat value, volatile matter and ash content of coal powder in different combustion layers under various working conditions, it was found that the difference in heat value and ash content is the main factor influencing combustion characteristics. Based on the actual operating conditions of the coal-fired power plant, a more realistic set of coal blending scenarios was established, and numerical simulations and on-site experiments were conducted, which showed that the overall combustion characteristics of coal powder under each working condition followed the aforementioned rules, and the carbon content in the fly ash reduced compared with the original operating conditions. Finally, the coal blending principles were established, which take into full consideration of the actual operating conditions of the power plant, recommend high-quality coal with low ash content to be used in the upper and middle layers of the combustion chamber while poor-quality coal with high ash content should be used in the middle and lower layers of combustion burner.