A multiscale geometric and computational model containing roughness unit was developed to study the effect of surface roughness on the fluid film pressure in microchannels of mechanical seals with a laser-textured porous seal face. By solving the N-S equation with numerical simulation methods of FLUNT, the effect of the fluid film pressure distribution by the height, distance and distribution position of roughness was investigated. The results indicate that both surface roughness and micro pores have the fluid dynamic effects. Roughness unit located in the rotational ring has more obvious influence on the fluid dynamic effects of the micro pores than that in the stationary ring. With the decrease of roughness unit density, the fluid dynamic effects in the rotational ring is enhanced, and the influence scope of dynamic effects is extended from the rotational ring surface to the stationary ring surface in the film thickness direction. Smaller or larger relative surface roughness has less influence on the film pressure distribution of stationary ring, but middling relative surface roughness will greatly affect the film pressure distribution of stationary ring, because it generates relative stronger dynamic effects and has larger influence scope, resulting in the reduction and even disappear of the low pressure area in the stationary ring generated by the micro pores.