Abstract: This paper presents a broadband, high-gain, and highly selective metasurface antenna designed under the framework of characteristic mode theory(CMT). The antenna adopts a multilayer structure comprising a Y-shaped microstrip feed line, a ground plane, a metasurface, and two dielectric substrates. The metasurface structure is optimized through characteristic mode analysis(CMA), enabling the modal significance coefficient(MS) of the main radiation mode to rapidly approach zero at high frequencies, thereby creating a high-frequency radiation null. Non-uniform rectangular patches are loaded on the metasurface to enhance gain, while a Y-shaped microstrip feed line is employed to further extend the bandwidth. Additionally, six complementary split-ring resonators (CSRRs) are etched into the ground plane to introduce a low-frequency radiation null by leveraging their resonant properties, thereby improving out-of-band suppression. Measured results show that the antenna achieves an impedance bandwidth of 29.1% (4.82-6.46 GHz) below -10dB, with out-of-band radiation suppression exceeding 14 dB. The peak gain within the passband reaches 10.58 dBi, the radiation efficiency is up to 85%, and the gain remains stable across the operating band.