The invention discloses a composite solid polymer electrolyte and a preparing method and application thereof. The composite solid polymer electrolyte is prepared from a gadoleic acid ester monomer, a porous supporting matrix, lithium salt, an additive and an initiator, wherein the mass ratio of the gadoleic acid ester monomer to the additive is (1-10):1, the lithium salt accounts for 5-30% of the total mass of the gadoleic acid ester monomer and the additive, and the mass ratio of the initiator to the gadoleic acid ester monomer is (1-10):100. The composite solid polymer electrolyte prepared with the method has the advantages of being large in ion conductivity, high in mechanical strength, easy to form, stable in interface contact, high in electrochemical stability and easy to prepare and can be used for forming solid lithium ion secondary batteries, and the prepared solid lithium ion secondary batteries are high in safety, energy density and production efficiency.