Abstract: The fly ash in the flue gas of coal-fired power station deposits a strong scale on the surface of the SCR catalyst,which is an important reason of catalyst deactivation. The ultrasonic resonance combined with steam purging method is usually used to clean the strong scale layer on the catalyst surface in time,However,due to the deposition of coal-fired fly ash on the surface of the catalyst,the formation of the strong scale layer is relatively stable,which results in the effect of ash removal under high-energy operation in the field not obvious.To better study the influence of fly ash deposition and agglomeration on the surface of the catalyst on the formation of strong scale,the stainless steel sampling tray was placed on the windward side of the three-layer catalyst layer for sampling by means of the field minor repair period and the thermal performance test of the SCR retrofit project.The samples of the three-layer SCR catalyst scale were characterized by atomic force microscopy( AFM).The black and white binary images were obtained by Matlab pattern processing by setting appropriate threshold values,and the box size was obtained by information entropy method to analyze the influence of multi-fractal scale interval.The results show that as the elevation zone of the stratification layer of the flue gas gradually becomes fragmented,the granularity of the deposited fly ash gradually becomes uniform,and the multi-fractal parameters have scale invariance with the increase of the flue gas flow.Meanwhile,the associated fractal dimension D_2 gradually closes to the Hausdorff fractal dimension D_0 with the increase of the flue gas flow.According to the variation property of weighted moment,it is found that the microscopic particles of the slab layer gradually change from loose non-uniformity to density uniformity with the increase of the flue gas flow.According to the multi-fractal spectrum symmetry of the three-layer slab layer,it is found that with the extension of the flue gas flow,the multi-fractal spectrum width Δα of the catalyst scale layer increases obviously,the surface roughness of the corresponding scale layer increases gradually,and the distribution of singular peaks on the surface of the scale layer is more obvious.The dominant factors of scale layer formation by fly ash deposition on the surface of the catalyst is mainly due to the probability distribution point of small particles and small elevation.The research shows that with the increase of flue gas flow,the multi-fractal characteristics of the scale layer on the catalyst surface become obvious,the particle size of the deposited fly ash becomes smaller,and the uniformity of the harden layer increases,and the elevation clustering feature of the micro-topological surface disappears,which provide a theoretical basis for on-site ash removal operations such as ultrasonic resonance and steam purge.