Abstract: In the process of the trenchless horizontal directional drilling (HDD) crossing installation, proper control on the undercrossing trajectory is needed for fear of drilling into the existed pipeline in order to reduce engineering risks. Among which, however, the construction of pilot hole is the first and foremost workstage of it. In this paper, the performance parameters of drilling tools together with stratum condition factors are analyzed to build a computation model of curvature radius and the inclination strength when pilot guide bit deflects into the stratum. The kick off point(KOP) and deflecting distance required to reach the preseted depth can be calculated by the computation model in the entrance; then in the exit, deflecting distance will be calculated by given KOP. This will ensure the bit drilling out of soil smoothly to the point and reach the goal of trajectory controlling. Finally, the practical applicability of this technique will be further proved in engineering practice.