An iterative continuation method for three-component magnetic field is presented.It aims at downward continuation of three-component magnetic field.The fields on those points with same horizontal coordinates are related by differential calculus.As divergence and rotation of three-component magnetic field are taken into consideration,vertical derivatives can be described with horizontal derivatives.As three-component magnetic field on an upper plane is known,magnetic fields on lower planes can be computed with an iterative procedure.Efficiency and accuracy are illustrated by applications.