A substrate integrated waveguide (SIW) Hplane horn antenna with wide band and high fronttoback ratio (FTBR) was designed. In order to improve the matching condition between horn aperture and free space, an integrated solution is suggested, which is based on a semicircular dielectric substrate and a metal piece along the horn aperture. The size of the antenna is 16λ×32λ mm, the impedance bandwidth is broaden and the FTBR is improved. The simulated results show that the proposed antenna operates with 21% impedance bandwidth ranging from 228 GHz to 282 GHz with S11 below -10 dB. The FTBR is greater than 13 dB at the frequency from 23 GHz to 26 GHz. The maximum gain of this antenna can reach 122 dB and the gain which have a considerable character can keep stable in a wide frequency range. Compared with other SIW horn antennas, this antenna has a simpler structure, and it is better in terms of wide bandwidth, higher gains and FTBR.