To address the demands and challenges of rapidly advancing millimeter-wave radar technology for broadband and high-gain antenna design, we propose a novel hourglass-shaped slot structure. We develope a W-band high-gain broadband substrate integrated waveguide (SIW) antenna with a multiresonant hourglass-shaped slot array, which is optimized using genetic algorithms to balance the impedance bandwidth and radiation efficiency. The simulation results indicate that the SIW slot array excited multiple resonant frequencies, leading to bandwidth expansion. The antenna achieves a -10 dB impedance bandwidth from 76.32 to -100.39 GHz, with a relative bandwidth of 27.24% and coverage of 68.8% of the W-band. In addition, the radiation efficiency is 93%, with a maximum gain of 19.6 dBi. Despite its small size, the antenna offers considerable improvements in the gain and efficiency. The voltage standing wave ratio at the resonant frequency point is less than 1.2, and the return loss is below -20 dB. This antenna features a wide-frequency band, low profile, high gain, and ease of integration, making it suitable for wireless communication and millimeter-wave applications.