The invention discloses a retreating time analysis method for losing the ultimate heat sink of nuclear power unit power operation. The retreating time analysis method comprises the steps that the processing strategy and the key links of the SOP procedure for the accident of losing the ultimate heat sink of unit power operation are analyzed; modeling of the key links is performed, sensitivity analysis is performed on the main heat load of the CCW system based on the established model and the removing order of the main heat load of the key links is determined according to the result of sensitivity analysis; and the main time parameters of the unit in the retreating process are calculated based on the accident processing course and the removing order of the main heat load and the retreating time of the unit under different situations is obtained. Prediction of the unit retreating time can be realized so that the method can be used for nuclear power unit accident decision, maintenance strategy formulation and state oriented accident procedure optimization; and the method can also be used for the field of unit cold source design and beyond design basis accident safety analysis.