Aiming at the problem of poor temperature resistance of drilling fluid due to the high temperature degradation of conventional modified starch filtration reducers，a filtration reducer STAR-AM with temperature resistance and viscosity increase was prepared by using natural starch as raw material，2-acrylamido-2-methylpropanesulfonic acid（AMPS）and acrylamide（AM） as grafting monomer through graft copolymerization method. The structure of the product was characterized by infrared spectroscopy. STAR-AM was compounded with flow modifier，plugging agent，shielding temporary plugging agent，lubricating defoamer and HCOONa to form a clay-free drilling fluid system with high temperature resistance. The temperature resistance， viscosity increase and fluid loss reduction performances of STAR-AM in base mud or drilling fluid system were evaluated respectively，and the reservoir protection effect of the drilling fluid was investigated. The results showed that STAR-AM prepared under the condition of starch（corn starch and tapioca starch in mass ratio 60∶40）and monomers（AMPS and AM in molar ratio 1∶ 2）in mass ratio 65∶35 had good performance of temperature resistance，viscosity increase and filtration reduction. When the dosage of STAR-AM was 10—25 g/L，the viscosity and shearing force of base mud significantly increased after aging at 150 ℃，and the API filtration greatly reduced. After adding formate，the temperature resistance of base mud increased from 150 ℃ to 170 ℃. When the dosage of STAR-AM was 25 g/L，the API filtration of the clay-free drilling fluid after aging at 170 ℃ was only 1.3 mL，and the HTHP filtration was only 10.2 mL，which was significantly better than that of conventional modified starch filtration reducers. It had excellent protection effect on reservoir，and the permeability recovery of core exceeded 95% ，which could be popularized and applied in the open-hole drilling and completion of ultra-deep horizontal well.