Coring and keeping the stability hole wall in complex formation are technical difficulties in geological drilling circles. In Wenchuan Earthquake Fault Scientific Drilling (WFSD), 5 boreholes were in the fault zone and the formation was extremely broken and soft. A new type of stepped bottom jetting anti-erosion core bit was designed, numerical analysis was made on bottom flow field by CFD technology to optimize drainage groove structure, mud properties and hydraulic parameters in order to fine the best combination to lay foundations for core recovery rate improvement, core blockage prevention and higher hole wall stability.