This paper presents a simple design of the wideband filtering antenna based on the square patch resonator via slit-loading. By properly incorporating two pairs of slots along both radiation edges of the square patch resonator, two additional resonant modes are produced around the conventional TM mode. Moreover, these two modes both perform inverse electric field at two sides of the etched slot, which provide the potential for creation of radiation nulls. In this context, by taking fully utilization of these two new modes together with the TMa wideband filtering antenna with three resonances and two radiation nulls is constructed. The simulated results indicate that the proposed antenna is operating from 3.34 to 3.66 GHz with a factional bandwidth of 9.14%. Besides, two zeros exist in the boresight gain frequency response at 3.29 and 3.75 GHz, leading to the high frequency selectivity in both passband edges.