A method for extracting a critical dimension of a semiconductor nanostructure. The method includes: 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 the 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 the corresponding electronic spectra database, simulation parameters corresponding to the simulation spectrum being the critical dimension of the semiconductor nanostructure to be extracted.