A method for calculating eccentricity of rotor assembly axis based on radial runout measurement comprises matrix characterization of data and calculation of relative runout value at each point, establishment of a spring equivalent model and calculation of contact force, eccentric direction and magnitude; calculation of relative runout value; establishment of a spring equivalent model to analyze the relationship between force and displacement in each phase of a contact process, and then an uneven contact force at each point is obtained; and determination of eccentricity is to determine the magnitude of eccentricity. Based on the measured radial runout data in production practice, this method realizes the prediction of eccentricity of axis before assembly, improves the coaxiality of rotors after assembly, and has important practical guiding significance for axis prediction as well as assembly phase adjustment and optimization in the assembly process of aero-engine rotor pieces.