Abstract: The excavation analysis of the bottom-hole three dimension stress field excavation is carried out on the two-stage bits of three diameter ratio ?190.5 mm×?311 mm,?215.9 mm×?311 mm,and ?241.3 mm×?311 mm in order to research the diameter ratio of pilot to enlarger.The analysis model,excavating methods,analyzing process,and analysis options of the bottom-hole stress field are applied during the research process.The distribution curve of the bottom-hole stress field is obtained,which shows that the radial stress of the ?241.3 mm×?311 mm bit is more completely released and is favorable for increasing the penetration speed.The difference between radial stress and circumferential stress of the ?215.9 mm×?311 mm bit gets smaller but it still has great potential to improve its penetration rate.The difference of radial stress and circumferential stress of the ?190.5 mm×?311 mm bit is small and the potential to increase penetration speed.Its optimum diameter ratio of pilot to enlarger is 0.7~0.8.