The invention relates to a method for determining an optimal value of the operation condition of a gas-steam combined cycle generating unit, and the method comprises the following steps: S1, building an online monitoring and economical operation system of the gas-steam combined cycle generating unit, and collecting the measuring point parameters of the unit; S2, carrying out the classification and screening of the operation conditions of the unit through employing a clustering algorithm, obtaining N stable operation condition classes, and distributing the measuring point parameters, which can reflect the operation condition of the unit, to different stable operation condition classes; S3, carrying out the data mining of the optimal values of the measuring point parameters for each stable operation condition class, obtaining the optimal value of each stable operation condition class, calculating the confidence of each optimal value, and determining the final preferred value of each stable operation condition class according to the confidence. Compared with the prior art, the method facilitates the improvement of economical operation level of the unit.