The invention discloses a composite membrane and a preparation method thereof and a battery comprising the composite membrane. The composite membrane comprises a polyolefin porous membrane layer and a polyimide porous membrane layer, wherein the composite membrane also comprises an adhesive layer; the adhesive layer is positioned between the polyimide porous membrane layer and the polyolefin porous membrane layer and tightly attached to the polyolefin porous membrane layer and the polyimide porous membrane layer; and the pore diameter distribution of a polyimide porous membrane comprises that: the pore volume of pores of which pore diameters are 50 to 300 nanometers accounts for over 75 percent of the total pore volume, and the pore volume of pores of which pore diameters are less than 50nanometers and more than 300 nanometers accounts for less than 25 percent of the total pore volume, and the pore diameter is measured by adopting a mercury intrusion method. A lithium ion battery manufactured by using the composite membrane as a battery membrane has improved service life and improved processing yield. Moreover, the composite membrane also has high thermal stability, and greatly improves the safety performance of the battery.