Abstract: In order to analyze the effect of segmental joint weak layers on deformation of long span cantilever-casting PC box girders，FE models with segmental joint weak layer elements were established based on the FE model considering construction processes of the assistant shipping channel bridge of humen bridge． Considering variations in thickness and material properties of segmental joint weak layer elements，the problem was studied with FEA． The results showed that deformation of segmental joint weak layers could lead to significant increment in cumulative deformation of long span cantilever-casting PC box girders; the shearing deformation of segmental joint weak layers was quite small，and this effect was mainly induced by bending deformation of segmental joint weak layers． Meanwhile，construction technology measure of chiselling was proposed to decrease the effect．