In order to reduce NO_x emission,technical retrofit of low NO_x combustion had been carried out for opposed-firing supercritical boiler with swirl burner in a power plant.After retrofit the mass concentration of NO_x emission declined by more than 50% compared with the test results before retrofit.But the serious slagging was found and research was conducted to analyse the reason of the slagging based on testing and analysing the characteristics of the new burner.The serious slagging problem was solved by a series of experiments such as changing the swirl intensity of secondary air,fineness of pulverized coal,oxygen volumetric fraction and flow rate of over-fire air (OFA) etc.