The invention provides a modified separator, a preparation method thereof and a 
lithium ion battery. The modified separator comprises a separator substrate and a 
polymer coating layer, wherein the 
polymer coating layer is arranged on a surface of the separator substrate and comprises a 
polymer, and a side link of the polymer is substituted by 
lithium sulfonate. The surface of the separator substrate in the modified separator comprises the 
polymer coating layer, the 
polymer coating layer comprises the 
lithium sulfonate group, the modified separator has a function of conducting lithium ions, themobility number of the lithium ions in an 
electrolyte can be increased when the modified separator is used in the 
lithium ion battery, the 
AC resistance value of the modified separator is reduced, the 
concentration polarization phenomenon is reduced, so that the growth of lithium dendrites of a battery negative 
electrode is reduced or prevented, the short-circuit risk of the battery is reduced, the safety and the rate performance of the 
lithium ion battery are effectively improved, and the cycle service lifetime of the 
lithium ion battery is prolonged. According to the preparation method of the modified separator, the 
polymer coating layer of which the side link is substituted by the lithium 
sulfonate is coated on the surface of the separator substrate, so that the modified separator hasrelatively low substitution degree in a polymer and also has relatively high lithium 
ion mobility, and the effect of reducing the lithium dendrites is achieved.