The actual load spectrum applied on structures is the key of the fatigue life mo nitoring. This paper presents a method of physical prototype centered assessment for fatigue life of structure based on on-line monitoring data. Firstly the lo ad spectrums of fatigue hot regions are re corded by some sensors, then the load of the structure is inversely computed by physical prototype, finally the fatigue life of the structure is estimated a ccording to the measured stress and strain history. A specimen of lug is tested and the strains of eig ht points near the hole are recorded, and then the fatigue life is estimated. The results show the feasibility and effectiveness of the proposed method.