Based on the topology optimization and genetic algorithm (GA), a kind of metamaterial absorbers with wideband, polarizationinsensitive and lightweight is proposed and applied to radar cross section (RCS) reduction of antennas by using resistive frequency selective surface (RFSS). Specific absorption of absorbers in range of frequency band 6.3~20 GHz is greater than 90% and this can remain above 80% when the incident angle increases to 50°under conditions of TE and TM polarizationindependent. The absorber is loaded to the microstrip antenna with a cover plate form, and the reduction of antennas RCS is greater than 3 dBsm in range of frequency band 6.3~20 GHz, and is greater than 10 dBsm in range of frequency band 10.37~10.90 GHz under conditions that the original radiation characteristic of antennas is unchanged. As an application example of this absorber, a lowRCS microstrip patch antenna is designed by mounting the absorber on substrate of the antenna. The RFSS absorber is made of carbon films attached on lightweight foam. The area density is only 0.35 g/cm3. The microstrip antenna designed is low RCS, lightweight, and lowprofile, and such antennas possess importance application values in stealthy technologies.