The physical mechanism of draw resonance of polymer fluids in melt spinning has been studied. It is proposed, according to the cross-sectional area dependence of the local draw ratio of the filament along the spinline, that filament drawing can be divided into three modes which are given in the main text. In particular, it is found that the nonuniformizing draw mode is the necessary condition of draw resonance. The mechanism is certified through the analytical solution of isothermal melt spinning under uniform spinning stress and the critical draw ratio of isothermal and uniform tension melt spinning. The mechanism was employed to analyze the promotive and suppressive factors of draw resonance in a real spinning system and the development of filament unevenness along the spinline.