In order to overcome the difficulty in maintaining the instability of drilling fluids after high calcium contamination ，ahigh temperature resistant and high calcium tolerant fluid loss agent DF-1 was synthesized using2-acrylamide-2-methyl propane sulfonic acid （AMPS）， acrylamide（AM）and diallyl dimethyl ammonium chloride （DMDAAC）monomers. The optimum synthesis condition was obtained as follows: 4.1∶2.9∶3.0 the monomer mass ratio of AMPS ∶DMDAAC∶AM，0.3% initiator ammonium persulfate ，7 pH value ，40℃reaction temperature and 3 hrs reaction time. Laboratory evolution experiment results showed that under 40000 mg/L CaCl₂ contamination and hot rolling16hrs at 150℃，basic mud with adding DF-1 could maintain the apparent viscosity in 9 mPa·s and the API fluid loss in 16mL；and that of basic mud without aging was 7.5 mPa·s and 30mL，respectively. The mechanism of DF- 1 was revealed by IR、TGA、SEM and EDS analyses. The IR analysis result showed that the polymer contained —CONH₂ and quaternary ammonium group ，which could help it to strongly absorb on bentonite by hydrogen and ionic bonds. TGA analysis result showed that DF- 1 had excellent thermal stability within300℃. EDS and SEM analysis results indicated that the relative calcium ratio of mud API cake in calcium contamination could reduce from 15.96% to 3.77% after adding DF-1，which attested that DF-1 was able to effectively decrease the ion exchange absorption of calcium on bentonite，thereby could prevent the bentonite particles from being coagulation and form a dense cake.