In order to realize the requirement of wide field of view (FOV) when used to blind zone exploration, collision warning and so on for Vehicleborne millimeter wave radar system, a broad beamwidth microstrip antenna array was designed. The planar microstrip antenna was used for basic radiating element. Antenna array was formed with 3 element to improve the gain. Antenna array was back fed with coaxial cable. With specific feed network, made the ratio of feed power was 05∶1∶05, which improved the gain and suppress the sidelobe level. In addition, the parasitic element and slot were added to the antenna array, which broaden the beam width and improved the impedance matching performance of the antenna. The antenna was verified by 3D electromagnetic simulation software HFSS. The simulation results show that the antenna array can work from 224 to 258 GHz with a maximum gain of 94 dB. The first sidelobe level is lower than -20 dB. The Eplane and Hplane halfpower beamwidth are 49°and 81°, respectively. The overall size of antenna is 175 mm×119 mm×1308 mm. It is useful for planar application and miniaturized of equipment.