The invention discloses a thermal power generating unit AGC performance evaluation index calculation and prediction method based on system identification. The thermal power generating unit AGC performance evaluation index calculation and prediction method comprises the steps of: establishing a first-order delay inertia model with a load command as an input and actual power generation power as an output based on actual operation data of a unit, and calculating unit AGC performance evaluation indexes by adopting parameters of the established model; and establishing a non-linear regression modelbetween different unit AGC index values and the corresponding unit AGC performance evaluation indexes K1, K2 and K3, and utilizing the established nonlinear regression model to realize predictive analysis on a unit AGC evaluation result. Through establishing the non-linear regression model between the unit AGC evaluation indexes and the unit AGC instructions, the function of predicting the unit AGC evaluation indexes is realized, and the defect that the unit AGC performance cannot be predicted at present is overcome.