Abstract: After the replacing liquid ammonia with urea,serious blockage problems occurred in the SCR ammonia injection systems of many units, leading to the loss of control of the ammonia injection process and even causing the units to shut down unexpectedly. Some researches expound on the operation problems and prevention measures of urea pyrolysis or urea hydrolysis from the operational perspective, but they do not conduct in - depth research on the blockage causes and solutions systematically.In order to solve the above - mentioned blockage problems, methods such as investigation, field measurement analysis, and CFD numerical simulation are used to study the blockage problems from the design perspective, analyze the causes, and propose corresponding optimization schemes. For the urea pyrolysis system, it is recommended to use cold primary air or air as the dilution air carrier. And appropriately increase the outlet air temperature of the pyrolysis furnace to solve the problem of urea crystallization. After the hot air enters the pyrolysis furnace, it will rotate violently, and the improvement effect of the uniformity of the NH₃ component at the outlet is limited even if a mixer is installed. For the urea hydrolysis system, it is recommended to optimize aspects such as the quality of the urea raw material, the source of the dilution air, and the operating state of the hydrolyzer to reduce the blockage caused by urea by - products such as biuret. In addition, it is recommended to optimize the structure of the ammonia injection pipeline and the ammonia injection grid to improve the system resistance distribution and avoid areas with too low flow velocity. On the one hand, this can alleviate the crystallization blockage of biuret, and on the other hand, it can improve the uniformity of ammonia injection control.