Belonging to the technical field of cement additives for oil-gas well cementing, the invention discloses a low hydration heat early strength cement fluid loss reducer for oil-gas well cementing and a preparation method. The fluid loss reducer is a copolymer taking acrylamide, 2-acrylamido-2-methyl propane sulfonic acid, acrylic acid, (methyl) acryloxyethyl-trimethylammonium chloride and allyl polyethenoxy ether as monomers. The allyl polyethenoxy ether has a molecular structure as CH2=CHCH2O(CH2CH2O)nH. The fluid loss reducer provided by the invention can make the fluid loss amount of low hydration heat early strength cement reach less than 50ml/30min at 20DEG C, ensures the safety of cementing operation, and improves the cementing quality.