Abstract: As definite integral is used for coordinate increment calculation of well track with constant tool face angle, it gets more difficult to find the solution to the problem of corresponding well track design. For example, the calculation takes long time and the convergence of the calculation method gets poor. Supposing the curved portion is a constant tool face angle curve, new mathematical models of well track design of 3D directional wells and sidetracked holes are established. By using the power series expanding method to calculate the definite integral, the solution to multivariate nonlinear equations is dimensionally transformed into binary nonlinear optimization. As a result, the numerical calculation gets less complicated remarkably. By using the numerical optimization method, the calculation saves more time, the stability of the method gets more stabilized, and the convergence is improved as a whole. A great number of examples show that the new calculating method can be used to find the solution to the design problem of well track with constant tool surface angle more rapidly and effectively. The power series expanding method, a very effective mathematical simplification method, is proved to be applicable to a series of calculating problems of well track with constant tool face angle.