In this article, highly selective filtennas with wide-stopband based on multifunctional slots loaded substrate-integrated cavity (SIC) are presented. Initially, the radiation characteristic of the half-mode patch unit is introduced. A nearly half size reduction and multiple high-order modes rejection are achieved by employing half-mode patch with metal via array arranged on the central symmetry axis, as compared to the conventional patch. Subsequently, by loading H-shaped slot into a rectangular SIC, effectively generating two gap-coupled half-mode patches. This arrangement fed by coplanar waveguide (CPW) structure enables the design of a second-order filtenna with three controllable radiation nulls (RNs). Moreover, H-shaped slot is divided into two C-shaped slots, resulting in a naturally introduced half-wavelength resonator. By leveraging the cross-coupling between different resonators, an extra RN and an extra in-band resonance are further generated, enhancing the frequency selectivity. Notedly, the two designs both exhibit improved stopband performance due to the utilization of half-mode patch radiators. The measured results are in good agreements with the simulated ones, providing compelling evidence for the feasibility of the proposed design methodology.