The long term deflection behavior of longspan continuous rigid frame bridges has often attracted the attentions in engineering field. Based on the moisture diffusion theory, the calculation method of differential shrinkage deflection was proposed for PC boxgirder bridges. Finite Element Method(FEM) model of 2D moisture field were established for typical cross sections of a case bridge. And the distribution law and the timevariation characteristics of moisture field were analyzed. Through the deformation coupling condition of moisture field, the drying shrinkage deflection of the bridge was calculated and analyzed. It was found that the differential shrinkage deformation was significant and great error would be with the conventional method. When considering the effect of differential shrinkage, the deflection in midspan was 3.1cm larger than that of the conventional method. Thus, the differential shrinkage was the main reason for long term deflection of longspan continuous rigid frame bridges.