As a particular phenomenon in the force displacement relationship of nonlinear energy sink (NES), tip effect is not found in other types of NES. In view of an inquiry into the reason of the tip effect of the orbital NES, and for a design of the track NES based on the tip effect, a theoretical research on the tip effect of NES has thus been carried out based on the formula derivation of the restoring force of NES, followed by an analysis of the tip effect of NES by using the free vibration of NES in undamped track and the forced vibration of NES in damped track. The results show that, with Fv being the main reason for the tip effect in the track NES, the decrease of Fv is more likely to occur than the decrease of Fa in the track NES, meanwhile the tip effect is stronger at the larger displacement. Before Fv reaches its maximum value, the absolute values of Fv and Fa are quantitatively similar, and because the signs of Fa and Fv are always opposite, Fd becomes the most representative term in the restoring force. Consequently, Fd can be used as the restoring force for the preliminary design of track NES.