The invention discloses a broadband flat plate array antenna, broadband can be achieved, and array antenna gain can be improved. According to the broadband flat plate array antenna, a coupling groove is used for stimulating two micro-strip subarrays, at the time, the impedance of a single micro-strip wire is high, a radiation paster can easily carry out high-impedance matching, accordingly the bandwidth of the micro-strip subarrays is widened, broadband is achieved, the relative bandwidth (Sll < -10 dB) of the feeding mode can reach 16%, 1 dB gain bandwidth can reach 14.6%, and common structure bandwidth is only about 6%. In addition, an H-shaped base chip is used for integrating a waveguide feeding network, the micro-strip subarrays are connected in parallel for feeding, along with increasing of array units, the bandwidth is hardly changed, broadband features can be kept in a large array, and array antenna gain can be improved gradually. In addition, the feeding network is placed below the micro-strip subarrays, extra circuit area cannot be increased, and array antenna miniaturization is promoted. The broadband flat plate array antenna is suitable for promotion and application in the technical field of microwave and millimeter wave antennas.