This study focuses on a passive noise abatement technique in a small contra-rotating fan, aiming at reducing the interaction noise between the two rotors through porous trailing edge (TE) treatment to the forward rotor. A preliminary design with fixed perforation parameters is experimentally investigated, and 6–7 dB overall noise reduction is achieved compared with baseline design under the same aerodynamic output. A three-dimensional (3D), full-wheel, unsteady-flow numerical simulation of the acoustic design is carried out to better understand the noise reduction mechanism. Comparisons of monitored unsteady forces acting on both the forward and the aft rotor between baseline and perforated fan indicate that such treatment reduces all the unsteady forces. Thus, it can be concluded that the noise reduction would be due to not only the mitigation of viscous wake of forward rotor before impinging upon the downstream blades but also the reduction of the response of the upstream rotor to the potential flow interaction with the downstream rotor. Furthermore, a parametric study in a selected range is conducted to minimize the adverse effect of aerodynamic unloading due to TE perforation and to improve the acoustic benefit. The parameters in the parametric study include perforation ratio, aperture diameter, and perforation distribution. Trends are deducted from this, and it is recommended that there exists an optimal perforation ratio; the smallest possible aperture diameter and the decreasing perforation ratio distribution away from the blade TE should be selected in consideration of both aerodynamic and acoustic effects.