Drill bits with PDC（Polycrystalline Diamond Compact）are used widely in geological exploration ， oil and gas drilling and other corresponding fields and good technical， and economic results have been obtained. However， the research on the rock fragmentation process and relationship with drilling parameters in not enough and it affects the further increase of the technical and economic indexes of the PDC bits. Russian drillers have made a lot of work in this field，including the bits，and obtained definite success and it worth us pay attention to. For example， they propose that rock fragmentation should be proceeded under volumetric fragmentation state as well as possible because the rock fragmentation mode is changed； If energy consumption of penetration per revolution of bit is included as an evaluation index， the evaluation system will be more scientific and more overall； Penetration per revolution of bit is not the higher the better， it has an optimum value. The value is depended from drilling parameters closely； it is put forward an idea of critical drilling parameters， meaning that while the product of bit weight and rotary speed is more， than the product， the temperature of the bit will rise drastically， the bit will wear seriously and any large quantity of drilling fluid cannot help resolve the problems. In this case it is necessary to take measures， adjust drilling parameters and avoid drilling process being in the critical state， so that drilling is in the normal state and the drilling can be continued.