In the research of terrain navigability, how to evaluate some terrain’s navigability is an important problem at all times when aircraft fly into the terrain in different directions. This paper sets up a mathematical narration (FBM) according to the terrain’s nature features. Through the analysis of the profile spectral moments and the surface spectral moments, a new measurement value of terrain feature—isotropy- coefficient is presented, which is based on the application of fractal geometry theory. This paper also derives the qualitative relation between matching probability of congruent terrain and different directions, which helps to improve the efficiency of navigability analysis.