Abstract: The burner is the key equipment to ensure the efficient and safe operation of pulverized coal boilers and low NO_x emission. Numerical simulations were carried out for a 300 MW Xinjiang Zhundong coal-fired boiler with a horizontal dense-lean bias combustion, focusing on the effects of the horizontal bias ratio at the burner exit on the slagging potential and NO_x emission. The results showed that the horizontal rich lean bias ratio at the burner exit directly affected the combustion temperature distribution and coal particle trajectory in the furnace. With the increase of the rich-lean bias ratio, the coal particle gradually moved to the center of the furnace, so the particle concentration distribution in the horizontal cross-section of the furnace showed a trend of high in the center and low in the surroundings. The oxidizing atmosphere became stronger, and the temperature was obviously lower in the near-wall layer. Thus, the high-temperature slagging of the membrane wall could be effectively mitigated. At the same time, the NO_x generation rate in the burner area gradually decreased. However, with the increase of rich-lean bias ratio, the NO_x concentration at the exit of the furnace showed a trend of first decreasing and then increasing. When the bias ratio was larger than 5∶1, the unburned carbon and the NO_x concentration at the furnace exit increased. According to the numerical simulation results, the optimal rich-lean bias ratio at the burner exit was 4∶1~5∶1, which could effectively prevent high-temperature slagging while reducing the NO_x emission at the furnace exit.