A method for gating in tomographic imaging system includes steps of: (a) performing a tomographic imaging on an object with a target moving periodically along a first axis (s) for acquiring projection images (Pθts); (b) obtaining projected curves (Cθts) by summing up pixel values along a direction of a second axis (t) perpendicular to the first axis (s) in each projection image (Pθts); (c) determining a target zone on the projection images (Pθts), wherein a central position on the first axis (s) of the target zone is corresponding to a position having the largest variation in the projected curves (Cθts) on the first axis (s); (d) calculating parameter values of pixel values in the target zones and obtaining a curve of a moving cycle of the target according to the parameter values; and (e) selecting the projection images (Pθts) under the same state in the moving cycle for image reconstruction according to the curve of the moving cycle of the target.