Drill bit anisotropy denotes the difference in drilling ability of a bit in the axial and lateral directions, and is an important factor impacting directional drilling efficiency. Quantitative evaluation of PDC bit anisotropy is important for well trajectory control and PDC bit optimization. Based on the structural parameters of PDC bits such as inner cone depth, outer structure height, back-rake angle, and gauge structure, the quantitative evaluation model for determining PDC bit anisotropy is established and a method for solving the model coefficient is given. This model was applied to evaluate bit anisotropy of PDC bit A in the PY35-X well in the South China Sea. The results were used to optimize the design of a new PDC bit, B type. The PDC bit B was then used in the 2450-2850 m section of offset well HZ25-X, with the inclination angle being satisfactorily controlled in the range between 1.2 and 1.95 degrees. Successful application of bit A in the offset well proves the validity of the evaluation model. The model of quantitative evaluation of PDC bit anisotropy helps to overcome the shortcomings of laboratory tests, like complicated test conditions and poor applicability. Basing on this model, the effect of structural parameters of PDC bits on the bit anisotropy index was discussed. It was shown that the bit anisotropy index increases as back rake angle, friction angle between the PDC bit and rock and bit size increase, and decreases as outer structure height, inner cone depth, and friction surface of the gauge increase. When drilling with a pendulum bottom hole assembly, it is recommended to use a PDC bit with high anisotropy index, which helps to minimize the tendency for the well to build angle due to strata deviating forces.