Combined the design case of using aqueduct to bridge main canal over Xiangpaigan ditch in Jiaokou district, considering the force displacement coordination of aqueduct body in two orientation, the beam bottom is elastically supported by every longitudinal beam in lateral calculation, the article applied practical space method to calculate the internal force of rectangular aqueduct of multi-longitudinal beams with cross rod,and then analyzed the influences of cross rod and primary beam stiffness on structural internal force. The result reveals that adding cross rod can decrease bottom stringers' bending moment but increase side rib' bending moment.Cross rods make the internal force of horizontal structure more uniform.The cross rod which is often called tie rod is actually subjected to compressing stress;with the increase of stiffness of vertical beams, the vertical deformation of aqueduct decreases,the bending moment of horizontal structure decreases; the uneven distribution of longitudinal beam's force decreases. The outcome can provide reference for the structure design of similar aqueduct.