Abstract: The prestressed steel and concrete composite beams having been widely used in engineering as a new transverse bearing component. The bending capacity is one of the most fundamental and important behaviors. The calculation formulas of elastic bearing capacity and deflection were derived considering the slip at the steel-concrete interface and the stress increments of prestressed tendons on the base of the elastic theory; and on the premise of plane section and complete composite action, the expressions of prestressed composite beam’s ultimate plastic capability which consider the stress increments of prestressed tendons were deduced, based on the simplified plastic theory. These foumulas can reflect the characteristics of the prestressed composite box girder. It turns out that the calculation values are in good agreement with the experimental results, which meet the engineering accuracy.