Several issues related to the study of magic springs are explored and clarified. A model of pre retracting force spring is established and its deformation law is derived. According to the motion theorem of the center of mass, the time for the spring to shrink to the original length, the equations of motion of its upper end and the speed formula are derived when the spring falls freely, and the applicable conditions of the contraction time formula in some current literatures are given. Research has shown that for the magic spring commonly used in demonstrations, during the contraction phase of free fall, its upper end does not move uniformly in a straight line