Limited by the flexibility of beam scheduling and receiving channel number, the range/azimuth two-dimensional (2D) scan is mainly used in the current diversity phased array radar for collision avoidance. Due to the lack of targets" height information, if there is a high/low target in front of the radar which does not affect the passage of the carrier, it will likely to cause false alarm. Aiming at this problem, this paper proposes an antenna layout and signal processing method based on the combination of MIMO and diversity phased array radar to realize the three-dimensional detection. Compared with the two-dimensional collision avoidance radar, this method can effectively realize 3D detection without increasing the number of radar receiving channels and the radar size. Compared with the 2D sparse array radar, the radar designed in this paper does not generate azimuth grating lobe, which is suitable for multi-target detection under complex background.