In view of the shortcomings of traditional waveguide-slot antennas with narrow bandwidth, many layers of cavities and high profile, a new low-profile and wide-band waveguide-slot antenna is designed. This design improves both the radiating element and the feeding network of the traditional scheme, increases the gap spacing, as well as achieves the effect of good broadband matching by reasonably increasing the allocation block. The increase of the slot spacing enables the same-layer design of the feeding network to be realized, thereby reducing the number of layers of the antenna and reducing the antenna section. In this paper, the HFSS simulation optimization of the new flat-panel antenna is carried out. The simulation results show that the relative bandwidth of the new flat-panel slot antenna can reach 15%, and there are only 2 layers, with the advantages of wide frequency band, less layers and low profile. The antenna can be used as the main solution for the future flat-panel slot antenna.