In this paper, aircraft equivalent single wheel load coefficientζ and adjacent wheel effect coefficient φ of airport flexible pavement structures are introduced, and the concepts and formula expressions of equivalent single wheel load are defined. Calculation methods of φ and ζ are introduced. Based on flexural tensile stress, strain at the bottom of structure layer and compressive strain on top of sub-grade, the rule of ζ with pavement structure parameters is analyzed when the main landing gear configuration of aircraft is dual wheel, dual tandem wheel or three axis wheel respectively. The paper suggests that in calculating ζ of flexural tensile strain at the bottom of asphalt layer and of flexural tensile stress at the bottom of rigid or semi-rigid base, the only one axle (or a row) load should be considered, and in calculating compressive strain on top of sub-grade, all adjacent wheels should be considered. At last, the approximate formulas of equivalent single wheel load coefficient for more than 40 kinds of aircraft based on flexural tensile stress, strain at the bottom of structure layer and compressive strain on top of sub-grade are given.