Skewly supported continuous box girders have special mechanical properties compared with common continuous box girders. Skew coefficient for stress of skewly supported three-span continuous box girder is introduced to reflect the decrease degree of the normal stresses at the cross section of the middle span of the box girder. The finite element models of the skewly supported three-span continuous box girders with different skew angles are established by applying the SHELL63 shell element in software ANSYS. The variation of the skew coefficient for stress at different calculation points of middle span cross section under concentrated load is analyzed and the corresponding influence law is revealed. Research results show that the skew coefficient for stress of top slab is different from that of bottom slab at the cross section of the skewly supported three-span continuous box girder. The skew coefficient for stress of top slab is smaller than that of bottom slab. Skew coefficient for stress decreases with the increasing of the skew angle. Because of the shear lag effect, the minimal skew coefficient for stress of flanges occurs at the free tip of cantilever slab while the maximal skew coefficient occurs at the intersection of web and flange slabs.