GPR profile is designed to the crossed grid in the tunnel, and in some special circumstances required along the tunnel cross section for the circular detection,The profile is a circle connected by start and end. Plane and spherical geologic structureare characterized by arc shape in the radar images. Based on space geometry relation of the circular line and detecting object, the relation formula of the reflected signal is derived and developed software, and the precise detection of geometric parameters of the object can be obtained.