The invention discloses an electrically-small third-order filter antenna with a good out-of-band rejection characteristic. The electrically-small third-order filter antenna belong to the field of filter and antenna cross technologies, and comprises a feeder portion, a resonator portion, a coupling line portion, a radiating patch portion, a dielectric substrate and a grounding plate. The electrically-small third-order filter antenna is based on two open-loop resonators with openings facing each other, and replaces a last resonator with a zigzag antenna comprising a sector, so that the antenna has energy radiation and resonance functions, thereby reducing integral size of a filter and the antenna, and making transmission loss between the filter and the antenna close to zero. Accordingly, the antenna serving as the last-order resonator has a function of out-of-band rejection. Further, a resonator is parallelly connected in the interior of a first resonator, so that bandwidth of the filter antenna is significantly increased while maintaining the original size unchanged. The electrically-small third-order filter antenna further has the advantages of compact structure, small electrical size and easy processing.