In order to develop small-sized nano-starch particle with high efficiency and low cost，ultrasonic treatment and sedimentation method were combined to prepare nano-starch particle by using the starch solution with a high concentration of 50 g/ L. And the effect of nano-starch particle on the performance of drilling fluid was studied. The results showed that the viscosity of starch solution with high concentration could be reduced by ultrasonic treatment，and the particle size distribution of nano-starch particle prepared by sedimentation method was 10—100 nm，mainly around 30 nm. Compared with the A-type structure of native starch，the formed nano-starch particle had C-type structure，and its solution could resist salt to 20 g/L NaCl at room temperature. With increasing dosage of nano-starch particle in base mud，the apparent viscosity，plastic viscosity and dynamic shearing force of drilling fluid increased，and the filtration decreased. Nano-starch particle could improve the temperature resistance of drilling fluid. The filtration of drilling fluid with the concentration of 10 g/L nano-starch particle was only 10.0 mL after aging at 150 °C for 16 h. Even if the aging time was increased to 72 h，the rheological properties of drilling fluid had a little change. The method of combining ultrasonic treatment and sedimentation method was easy to be operated，and the formed nano-starch particle additive of drilling fluid had excellent performance with a relatively low cost，which was easy to be popularized and applied.