This paper discussed the methods to analyse the bottom hole assembly in horizontal well and gave the non-linear dynamic governing equation of BHA as well as its initial and boundary conditions. A vector model of rock-bit interaction was established. A new concept of the effective drilling force was proposed. Based on the model and concept, some typical anisotropic formations were classified into 12 categories. Their anisotropic drilling characteristics could be presented in the equations and graphs. In a field example, the deflecting performance of a typical downhole motor assembly with the bent housing and two stabilizers in horizontal well was discussed in detail. The adjacent well distance scanning and plotting method and its engineering application were introduced in the last part.