To investigate the stress variation of reinforcement in beam under fatigue loads, in this paper, based on fatigue test results of 12 post-tensioned partially prestressed concrete beams, the changes of reinforced maximum, stress amplitude which are closely related with the fatigue failure of partially prestressed concrete beams are obtained. The experimental results suggest that the prestressed and non-prestressed reinforced are basically at the same ratio of development. It has been found that the key factors of affecting fatigue capability of partially prestressed concrete beams are stress amplitude of reinforcement. The bond degradation of reinforcement and reduction of effective prestressed stress lead to stress redistribution, which are the two main important reasons why prestressed and non-prestressed reinforced damages are not in desynchronized way with the increase of repeated loading. Finally, a calculation method of prestressed steel stress increment in a cracked section after decompression is proposed by the experimental analysis, and the calculation results are matched with the test results quite well.[JP]