Abstract: Respiratory motion is the primary source of error in the 4D precise radiation therapy.It’s a hot topic to analyze the organ deformation between different respiratory phases by registration of 4D images.Currently,3D deformable registration algorithm is commonly used in the research.The algorithm based on optical flow has high speed and high accuracy.Demons algorithm and symmetric gradients are applied to the calculation of original deformable force of 4D CT registration.It is demonstrated that Demons algorithm using symmetric gradients converges faster to results with less error.