Abstract: Ash accumulation is one of the critical problems affecting the performance and secure operation of SCR de-NO_x system in coal-fired power plants. Unfortunately, the fly ash concentration field is often neglected in traditional engineering design. In this paper, the ash accumulation states of a certain 600 MW coal-fired power plant was studied. The calculation model was established according to CFD method and the distribution of flue gas flow field and fly ash concentration in de-NO_x reactor was simulated. Also a structure optimization was established to improve the distribution of flue gas and fly ash. The results indicated that the distribution characters of fly ash decreases with the increase of particle size and density of fly ash particles. The high concentration and density of fly ash particles can easily lead to the clogging of the catalyst layer, which may lead to the serious problem. The distribution of flue gas flow field and fly ash should be demonstrated to ensure the safe and efficient operation of the device when the fly ash concentration field is designed.