A novel substrate-integrated waveguide (SIW) horn antenna with partially detached broad walls is designed and analyzed. The conventional SIW horn antenna suffers from narrow-operating bandwidth, which mainly results from the mismatch at the horn aperture between the substrate and the free space. However, the detachment of the broad walls makes the left substrate not only act as wave-guiding structure but also impedance transformer, which brings much size reduction in comparison with conventional SIW horn antenna. Moreover, the substrate with graded dielectric constants distributed in the propagating direction is realized through perforating air-vias with different diameters to further improve the radiation performance. Results indicate that the proposed antenna operates from 17.7 to 26.7 GHz with a nearly constant peak-radiating gain between 8 and 9 dBi at end-fire direction. Stable end-fire radiation patterns can be realized in the whole operating band. Above all, the proposed technology makes it compact to design SIW horn antenna of high performance without extra extension after the aperture of the radiating horn.