The invention discloses a trinary-copolymerized side-chained liquid crystal polymer and a preparation method thereof. Under the action of triggering of azodiisobutyronitrile, methacrylate monomers containing azobenzene structures and N-substituted maleimide monomers are heated for a thermal polymerization reaction with toluene as a solvent, a copolymer is obtained through precipitation in methanol, that is, the trinary-copolymerized side-chained liquid crystal polymer containing azobenzene monomers, maleimide and methyl methacrylate is obtained, and finally, a liquid crystal polymer film is obtained by dissolving the side-chained liquid crystal polymer in a toluene solution and performing spin coating. The advantage of photo-induced isomerization of azobenzene is kept, and defects of poor orientation transparency of photo-induced isomerization of azobenzene and poor heat resistance are made up by use of the optical inertia of methyl methacrylate and the rigid structures of the N-substituted maleimide monomers. The novel liquid crystal polymer film material has a very important application prospect in liquid crystal display technology.