The mechanical property of external prestressed beams is affected by many parameters. Based on experimental study on externally strengthened prestressed box beams, a three-dimensional finite element analysis model was built in ANSYS. The calculation results was proved to be correct by being compared with experimental results. The effect of some parameters on deflection and stress in externally strengthened box beam was deeply analyzed in serviceability stage and ultimate status. The study shows that different original status of strengthened beam may affect the deflection and stress in different degree. The parameters change, such as effective prestress, concrete strength, the area and depth of external tendons and displacement coupling character for contact node, will result in a little difference in deflection and stress in serviceability stage, but obvious in ultimate status. For box beams in the model, changing the tendons lateral distance to the web results in litlle change in deflection and stress. No matter it is inside or outside the box at cross section.