Abstract: Based on the experimental resultof4 three-span externally pre-stressed concrete continuous beamswith different pre-stress degrees, reinforcement ratios and sections, the stress increment of externally pre-stressed tendonswas investigated. The result shows that the stress increment is directly proportional to the deflection atmid-section in the middle span, and the relation among the stress incremen,t the deflection and the load may be described using a model of tri-broken line. In this mode,l crackings of themid-section and themid-support section are taken as its turning points. As a resul,t a simplified method of calculating the stress increment based on the elastic theory was proposed, the calculated resultbeing accordedwith thatof a non-linear analysis.