Ultra-small section circle lean coal boiler has problems such as serious slagging and high temperature corrosion. In order toadapt to the current coal market diversification and boiler flexibility operation and other needs,it is urgent to improve its adaptabilityto complex coal blending. Taking the 300 MW ultra-small section of a power plant as the research object,the coal adaptive combustion modification was carried out. Firstly,CFD numerical simulation was used to evaluate the combustion problems of the original combustion system of the boiler when mixed with bituminous coal,then the modification scheme of the combustion system was put forward accordingly: by increasing the burner distance to reduce the heat load of the burner area,reducing the imaginary circle to solve the flame brushwall,increasing the burn out air rate to achieve deep air classification and other measures,the coal type adaptability of the boiler could beexpanded. Then CFD numerical simulation was used to determine the best performance parameters of the combustion modification scheme,and predict the combustion state of the boiler under 67% and 100% low ash bituminous coal after combustion modification. The effectiveness of the combustion modification was verified through the engineering test. The results show that the 300 MW ultra-small section circle-fired boiler can be mixed with 20% bituminous coal to adapt to burn 67%-100% low ash melting point bituminous coal,so the adaptabilityof coal species can be effectively enhanced. According to the official assessment and test, the efficiency of the boiler in thedesigned coal condition is improved by 2.25%,NO emission rate is reduced by 40%-48%,and there is no obvious slagging and high temperature corrosion problems in the furnace. The combustion modification scheme provides guidance for the adaptability of bituminous coal.