Low viscosity copolymer fluid loss additive for drilling fluid is a temperature resistant and salt resistant polymer fluid loss additive with ultra-low viscosity effect, which is synthesized from AA, AMPS and DMDAAC as main raw materials, water as solvent, and redox initiator system. PANS not only effectively reduces drilling fluid filtration, improves mud cake quality, has strong resistance to temperature and salt calcium pollution, but also maintains a low viscosity effect by strictly controlling the molecular weight of the product, making it particularly suitable for drilling in fresh water, saline water, and seawater