In HDD, the tension and torque acting on the bit are important foundations for the design of drilling and construction. The tension-torque mathematical model is established by the mechanical analysis on extrusion type reaming bit and the factors which affect the results of the model are analyzed entirely. The model was tested according to the construction data, the results agreed with field data well. Therefore, the model plays an important role for construction design optimization on HDD with extrusion type reaming bit.