A method of flatness measurement of large annular planes was designed based on three-point method. By measuring the local curvature of the annular, the overall curve was connected and the flatness measurement of the whole annular was obtained. The measuring principle of three-point method was introduced, as well as the application of three-point method on measurement of large annular planes. The factors of zero error which impact the measurement result were analyzed. Experiments show that the method could solve the problem of flatness measurement of large annular flat parts.