1. A method for extracting a critical dimension of a semiconductor nanostructure, the method comprising: 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 optimum simulation spectrum being the critical dimension of the semiconductor nanostructure to be extracted; wherein said electronic spectra database is obtained by: a) dividing said value range for each parameter to be extracted into multiple sub-value ranges; b) choosing one sub-value range from said sub-value ranges whereby generating a sub-parameter value combination; and c) extracting multiple groups of discrete values for sub-parameter value ranges in each sub-parameter value combination, employing forward optical modeling program to generate a corresponding simulation spectrum for said discrete values, a simulation spectrum corresponding to all discrete points in each sub-parameter value combination being an electronic spectra database.