Abstract: In this paper, energy method is used to study the change of the secondary moment of prestressed concrete continuous beams during loading. In the same time, comparison is made with the non-prestressed concrete continuous beams. The question whether the secondary moment is changing while loading is answered. It shows that the secondary moment is unaltered when increment method is adopted to study the beams. With further increase of load, the effect of the secondary moment begins to emerge. It is reflected in the change of the stiffness of the beam sections.