In order to meet the needs of millimeter wave radar system, a design method of microstrip broadband slot antenna suitable for millimeter wave band is researched. By using the structure of defected ground and "windowing" on the ground plate, good impedance characteristic is obtained at the expense of partial gain, which solves the problem of narrow band of traditional microstrip antenna. An ellipsoidal dielectric lens is designed to improve the gain characteristics of a microstrip slot antenna. Through simulation and measurement, the passband S11 is less than-10db in the 75GHz-90GHz frequency band, and the impedance bandwidth is greater than 30%. The 3dB beam width of the two main planes is 10 ° and 11 ° at center frequency of 80GHz. The characteristics of far field pattern are in good agreement. Dielectric lens increases antenna gain from 3.5db to 19.69db. The antenna has the characteristics of wide bandwidth and high gain, which can be used for long-distance millimeter wave radar detection.