1. A method for extracting a critical dimension of a semiconductor nanostructure, the method comprising the following steps: 1) determining a value range for each parameter to be extracted, whereby generating an electronic spectra database, and employing training spectra and support vector machine (SVM) training networks for training of SVMs; 2) employing said SVMs after training to map measured spectra to yield a corresponding electronic spectra database; and 3) employing a searching algorithm to search for an optimum simulation spectrum in said corresponding electronic spectra database, simulation parameters corresponding to said simulation spectrum being the critical dimension of the semiconductor nanostructure to be extracted.