Thermal viscosity-reduction-free oil well cement fluid loss agent as well as preparation method and application thereof

The invention relates to a thermal viscosity-reduction-free oil well cement fluid loss agent as well as a preparation method and application thereof. The fluid loss agent comprises the following components in parts by weight: 100 parts of water; 6 to 10 parts of 2-acrylamido-2-methyl-1-propanesulfonic acid; 6 to 10 parts of N,N-dimethylacrylamide; 2 to 6 parts of maleic acid; 1-5 parts of acrylic acid; 1 to 5 parts of long-chain alkyl acrylate; 5-6 parts of sodium hydroxide; 0.1 to 0.3 part of ammonium persulfate; and 0.1 to 0.3 part of sodium hydrogen sulfite. The thermal viscosity-reduction-free oil well cement fluid loss agent provided by the invention has moderate slurry (cement slurry) viscosity in a temperature range of 20-160 DEG C, and has good fluid loss reduction property; meanwhile, a long-chain hydrophobic monomer, namely the long-chain alkyl acrylate, is introduced into the components, so that the viscosity of hot slurry is slightly increased at a high temperature, and the problem that the slurry