Due to geographical factors and historical reasons, urban underground circular cavity "many", "dense", "chaotic" situation is more and more serious, geological radar circular cavity scanning image analysis is not clear. Based on the whole process of geological radar scanning and detection, the whole process of radar scanning underground circular cavity is divided into three stages. The relationship between horizontal distance and echo delay in each stage is established. The changes of circular cavity geological radar scanning images under different buried depths and different radii are systematically analyzed. Combined with the detection example, the scientific explanation of the imaging mechanism of underground circular cavity geological radar is realized. It is concluded that the left side of the symmetric axis of the underground circular cavity radar scanning image is a monotonically decreasing continuous concave function, and the thickness increases from constant to stable. The right side of the symmetric axis is a monotonically increasing continuous concave function, and the thickness decreases from stable to constant. With the increase of buried depth and radius, the curvature of the curve decreases, and the curve image tends to be relaxed. The characteristics of the curve reflect the trend of increasing openness.