Abstract: Externally prestressed steel-concrete composite beams share the features of prestressed steel structures, prestressed concrete structures and non-prestressed steel-concrete composite beams. The theoretical model to compute the long-term deflection of externally prestressed steel-concrete composite beams was deduced in this paper, which considered the effects of steel bars in concrete slab, concrete shrinkage and creep, and tendon relaxation. The theoretical model was firstly validated through a comparison between the computed results with the test results. Then factors affecting the long-term deflections of externally prestressed steel-concrete composite beams were discussed, including the age to tension, initial effective pretension, normal longitudinal reinforcement ratio, concrete slab width, steel beam height, environmental relative humidity, concrete strength and prestressing tendon types. The results showed that the existing of externally prestress reduced the additional deflection of composite beam by an amount of 28.9% when the external prestress countervailed the deflection produced by dead loads. Besides, a larger initial effective pretension can decrease the ultimate additional deflection at the mid-span. This study provides useful references for the design and deflection control of externally prestressed steel-concrete composite beams.