Objective To study the effect of the single grain’s geometric features for the material separation on cut root by simulation and get the critical grinding parameter values of brittle-ductile transition. Methods For the Ti6Al4V alloy, the thermal-force coupling plane simulation model was established through the analysis of single grain scratch to study the relationship of the effective flow stress along with the single grain edge radius and the grinding depth in the end of the cutting chip. Results On the grain’s blade edge, the critical phenomenon of brittle-ductile transition appeared when edge radius r was 0.1 μm and grinding depth h was about 0.02 μm; when h was >0.3 μm, the material’s plastic removal might be removed, which should be promoted by the grinding heat effect. When the radius of cutting edge r was 1 μm and the grinding depth was h was 0.2～3 μm, the minimum value of effective flow stress was 948.479 MPa. At this time, there was no material plastic flow at the grinding blade edge, and the grinding thermal effect was not obvious. When edge radius r was 10 μm and grinding depth h was about 2～30 μm, the minimum effective flow stress was 716.351 MPa and the maximum value 763.59 MPa, and the chips on blade edge were generated in plastic flow, which was promoted by the grinding heat effect. Conclusion The threshold range of plastic flow and the critical value of brittle-plastic transition are got in simulation process, and the effect of the grinding thermal effect on the formation of the chip is also obtained. Use of grains with edge radius of 0.1 μm or about 10 μm and proper increase of grinding depth can realize the plastic flow of the material, reduce the impact on the grain and improve the grinding efficiency.