A method of gating using an X-ray cone beam tomographic imaging system, said method comprising: (a) performing a tomographic imaging on an object for acquiring a plurality of projection images at different projection angles (θ) by the X-ray tomographic imaging system, wherein a target of the object moves periodically; (b) computing a projected position (t,s) of the target on each of the projection images, wherein the projected position (t,s) is a center of a target zone on each of the projection images; (c) calculating a parameter value of pixel values in the target zone on each of the projection images, and obtaining a curve of a moving cycle of the target according to the parameter values of the projection images; and (d) selecting the projection images under the same state in the moving cycle for image reconstruction according to the curve of the moving cycle of the target, wherein when the tomographic imaging is performed, an X-ray source and a detector are regularly rotated relative to a first axis for acquiring the projection images at the different projection angles (θ), and the step (b) comprises substeps of: acquiring the projected positions (t,s) of the target on any two of the plurality of projection images; according to the projected positions (t,s) on the two projection images and the corresponding projection angles (θ) and cone angles (γ), calculating an actual position (x,y,z) of the target in the three-dimensional space, wherein the first axis is perpendicular to a second axis, and the cone angle (γ) is an included angle between the second axis and a connecting line of the X-ray source and the target; and according to the actual position and the corresponding projection angle (θ) and cone angle (γ), calculating the projected positions (t,s) on the rest of the projection images. (a) performing a tomographic imaging on an object for acquiring a plurality of projection images at different projection angles (θ) by the X-ray tomographic imaging system, wherein a target of the object moves periodically; (b) computing a projected position (t,s) of the target on each of the projection images, wherein the projected position (t,s) is a center of a target zone on each of the projection images; (c) calculating a parameter value of pixel values in the target zone on each of the projection images, and obtaining a curve of a moving cycle of the target according to the parameter values of the projection images; and (d) selecting the projection images under the same state in the moving cycle for image reconstruction according to the curve of the moving cycle of the target, wherein when the tomographic imaging is performed, an X-ray source and a detector are regularly rotated relative to a first axis for acquiring the projection images at the different projection angles (θ), and the step (b) comprises substeps of: acquiring the projected positions (t,s) of the target on any two of the plurality of projection images; according to the projected positions (t,s) on the two projection images and the corresponding projection angles (θ) and cone angles (γ), calculating an actual position (x,y,z) of the target in the three-dimensional space, wherein the first axis is perpendicular to a second axis, and the cone angle (γ) is an included angle between the second axis and a connecting line of the X-ray source and the target; and according to the actual position and the corresponding projection angle (θ) and cone angle (γ), calculating the projected positions (t,s) on the rest of the projection images. acquiring the projected positions (t,s) of the target on any two of the plurality of projection images; according to the projected positions (t,s) on the two projection images and the corresponding projection angles (θ) and cone angles (γ), calculating an actual position (x,y,z) of the target in the three-dimensional space, wherein the first axis is perpendicular to a second axis, and the cone angle (γ) is an included angle between the second axis and a connecting line of the X-ray source and the target; and according to the actual position and the corresponding projection angle (θ) and cone angle (γ), calculating the projected positions (t,s) on the rest of the projection images.