Abstract: Torque is an important parameter during the drilling process, and the rock breaking rate is slow when weak torque that affects the rate of penetration, while high torque would result to drill string accident.The calculation methods for both torque values of various drill string on the surface and bit need to be summarized.For top drive system or rotary table of electric drive rigs, torque calculation of the surface drill string which is offered.For the bottom hole assembly with and without mud motor, the calculation methods of torque on the bit is proposed.The research results show that, under the condition of the certain voltage of electric motor and load resistance, the rotary speed has a corresponding torque of the surface drill string.If the bottom hole assembles without mud motor, the torque will decrease along the whole string from the surface to the bit.As to the bottom hole assembles with mud motor, the torque on the bit is the output torque of the mud motor under this load pressure drop.The output of mud motor will increase as long as this raised pressure drop value within the designed scope of the mud motor; the bit torque will be the output torque value of mud motor under the pressure drop once it is determined.Only if the value difference of torque on the surface and at the bit is no more than the torsional strength of any position of the drill string, the drill string twisted off accident won't happen down hole.The research results show that the proposed idea provides the theoretical guidance for choosing the rotary speed of the top drive system, or rotary table scientifically and matching drill bit and mud motor reasonably.