Abstract: A broadband substrate integrated waveguide (SIW) back-cavity slit antenna is designed based on a miniaturized metasurface. Firstly, several new resonant points are introduced based on the traditional H-shape slot to improve the bandwidth. The higher-order resonant modes in the sub-cavity are excited by the ground-based coplanar waveguide feeding network. A wide impedance bandwidth can be achieved by introducing the higher-order modes closer. By this means, the final bandwidth of the slot antenna can reach 20.7% (12.1-14.9 GHz). Secondly, the antenna is further improved by loading the miniaturized metasurface to solve the gain degradation in the high-frequency part. The metasurface is composed of four sequentially rotated 3×3 meta units and a metal via is etched on each unit to achieve miniaturization. Simulation and measured results demonstrate that the impedance bandwidth of the proposed antenna is 27.6% (11.25-14.85 GHz), and the peak gain is 9.4 dBi, in which the high-frequency band gain is improved by 1.7 dBi on average, and 2.3 dBi on maximum, compared with that of the SIW slot antenna without the metasurface. The designed antenna has good application scenarios in the field of communication.