In view of the regular problems in the airfield clearance condition evaluation, the paper calculates and analyzes the transitional surface range between terminal clearance zone and side clearance zone in detail. The paper indicates the shapes of the transitional surfaces in various zones, calculates the coordinates of the key dots, and puts forward a method of calculating the various dots in the line between transitional surface and inner level surface, and between conical surface and outer level surface. All these have provided a theoretical basis for determining exactly the position, the elevation and the superfluous height of obstacle in the airfield clearance zone.