A differentially-fed rectangular patch antenna under TM21 and TM03 modes is proposed to realize bandwidth enhancement and stable high gain. Firstly, two open slots along the median of patch are introduced to reconstruct the current distribution of TM21 mode so that its broadside radiation is strengthened greatly. Secondly, another central slot along with the two sided ones interrupt out-of-phase current of TM03 mode and make it circulate with little radiation, hence substantially reducing the sidelobe level (SLL). With the help of loading three cascaded slots and pins, the frequencies of the two modes are merged together, and so the antenna obtains an enhanced impedance bandwidth of 11.8% (2.24-2.52 GHz). Finally, the simulation results show that the proposed rectangular patch antenna achieves gain level as high as 13.8 dBi, which has little variation in the working bandwidth.