The backprojection filtering reconstruction algorithm is proposed based on M-line for the circle-plus-line trajectory. The concise and appropriate M-line selection method is proposed, combined with the geometrical characteristics of the trajectory. The backprojection integral interval is obtained through solving the R-line coordinates which pass through the reconstructed point on the M-line, and the proposed reconstruction formula is derived. It is proved that the R-line within the reconstruction region for the circle-plus-line trajectory is unique, and the shortest length of the needed line trajectory is solved. Experimental results show that the proposed method can improve reconstruction image quality in large cone-angle scanning.