Abstract: For a 300 MW thermal power unit, to solve the problem of the coking occurring in the tangentially fired boiler with coal blending, tests were conducted on the unit, and the combustion system was optimized and retrofitted. Test results show that the decrease of the coal mill output is caused by the the small area of the primary air nozzle, which leads to the coking of the water wall. Through improving the coal mill output with the decrease of the primary air velocity, the boiler load can be increased, while the concentration of the reducing atmosphere near the furnace wall can be decreased. The retrofit has a good effect, which can serve as a reference for the solution of the problem in similar units.