The frequency continuous extension to K/Ka band for electronic devices and wireless technology, as well the fast development of integrated electronic system have made the electromagnetic shielding design of electronic systems increasingly challenging. In this paper, a new shielding method which incorporates frequency selective surface (FSS) into electronic systems to replace the conventional heat dissipation holes array is proposed. The proposed shielding method can meet the requirements of both ventilation and electromagnetic shielding performance at 5G millimeter wave band. Based on the shielding effectiveness (SE) at the center point in the enclosure and the global shielding effectiveness, the influences of the arrangement of FSS cells and the polarization and incident angle of electromagnetic waves on SEs of the metallic enclosure are analyzed. The results show that the arrangement of FSS cells have little effects on the shielding performance of the metallic enclosure, and SE hardly changes with the polarization of the electromagnetic waves. The shielding effectiveness of the metallic enclosure with the FSS is about 30 dB in the frequency range of 23.0-25.5 GHz, which is 15 dB higher than that of the enclosure with the conventional heat dissipation holes array. This demonstrates that the method proposed in this paper has better electromagnetic shielding while keep good ventilation at the same time.