To investigate the effect of surface roughness on particle erosion in turbofan engines, this study focuses on a high-bypass-ratio turbofan engine. The Euler–Lagrange method was employed for particle tracking, and an equivalent roughness model was used to analyze particle erosion on fan blades under varying roughness conditions. The results show that under small-particle-size conditions, increased roughness causes particle–wall collision regions within the inner duct to concentrate near the shroud of the first-stage blades. As roughness increases, the average erosion rate density on the fan blade rises from 1.96?×?10???(kg·m?2·s?1) to 2.63?×?10???(kg·m?2·s?1), an increase of approximately 34%, while the area-averaged erosion rate density increases from a minimum of 2.28?×?10??(?kg·m?2·s?1) to a maximum of 2.42?×?10???(kg·m?2·s?1), representing an increase of about 6%. The average volume fraction of particles on the fan rotor blades increases with blade height; with higher roughness, its peak value rises from 5.85?×?10?? to 6.91?×?10??, an increase of approximately 18%. It is concluded that the roughness of the fan blade also influences particle impact angle and slip velocity, ultimately aggravating the original erosion pattern.