This paper introduces the technological process and operation mode of denitrification and ammonia supply in coal-fired power plants. The Selective Catalytic Reduction (SCR) flue gas denitrification technology of coal-fired power plant is a great significant to environmental protection, which denitrification efficiency is up to 90%. A coal-fired power plant SCR boiler flue gas denitrification device of ammonia controlling valves often block in the winter, and need stopping the device to carry out manual cleaning, that seriously affecting the normal operation of the SCR system and NOx emissions standards. Through the mothed of analyzing of the physical scale components of liquid ammonia quality, environmental temperature, equipment, material, jam, we had found the causes of the problem.... Ammonia injection valves plug is mainly made of carbon steel rusting and ammonia supplying of inner tube in oily volatile mixed cause. According to the actual situation of the equipment in the power plant, the operation mode of liquid ammonia evaporator was adjusted, the denitrification ammonia suppling pipeline was purged, the structure of ammonia injection regulating valve was reformed, and the stainless steel-pipeline was replaced. The retrofitting reason of ammonia injection regulating valves and their structure are introduced in detail, which is the key to solve the problem.