Abstract: Secondary moment concept is extended into post-elastic range by an equivalent linearization method using secant stiffness. An experimental study is conducted on four prestressed concrete continuous beams with unbonded tendons over entire loading. The results show that by an equivalent linearization method, the support reactions of a continuous beam in nonlinear stage can be decomposed into reactions due to tendon equivalent loads and to external forces, thereby separating the secondary moment and external load moment. The nonlinear stage secondary moment increases with the growth of tension in unbonded tendons. The reduction factor of secondary moment increases with the relative height of the compression zone of the section at an inner support. At the ultimate limit state, a nonlinear stage secondary moment may be greater than the initial secondary moment.