Abstract: To make use of the advantages of the higher drilling speed and the more footage,and to cut the shortage of the higher torque of PDC bit while drilling in the formation from soft to medium hard,this paper carried on an investigation in the PDC bit structure so as to improve the drilling speed in big size hole.Aim at the characteristics of the big size hole in deep well, the geometric and cutting structures of PDC bit was analysed and studied,and put forward the new structure theories that drill a small hole,and ream it.The new structure can reduce the broken power and change the distribution of cutting forces of the PDC bit,then hoisting the drilling-speed.According to the theories,the new design and manufacture of the bit were developed,and have obtained the good applied result in big size hole in the upper formation,which broaden the application scale of the PDC bit.The field test shows that the new technique lowers the torque,assigns the broken energy reasonably,increases the stability of bit and anti-vibration of the drill pipe,reduces the whirling tendency of bit,thus,reaches the level of higher drilling speed and better steering control.