Objective: To construct two different venous models, one with and one without a venous sinus, each containing two pairs of adjacent orthogonal venous valves. Using fluid-structure interaction (FSI) simulations, the study will investigate the influence of the venous sinus structure on blood flow in the lower limb venous system, and analyze the role of the venous sinus structure in the process of venous valve opening and closing. Methods: Based on ultrasound images and anatomical images of animals, two distinct 3D geometric models of the venous system will be constructed using modeling software. Using the immersed boundary method, fluid-structure interaction (FSI) simulations of venous valves will be performed to obtain hemodynamic parameters, such as changes in valve morphology, flow streamlines distribution, wall shear stress distribution, and Q-criterion intensity distribution. Results: The valve"s geometric opening area in the model with sinuses was larger than that in the model without sinuses. The distribution of blood streamlines demonstrated the existence of vortices in the sinus region and helical flow within the vessel. The same venous valve pair exhibited an asymmetric distribution of wall shear stress, with the region of high wall shear stress behind the venous valve and the Q-criterion intensity between the far and near end valves being smaller in the sinus model compared to the non-sinus model. Conclusion: The blood flow between two adjacent pairs of orthogonal venous valves exhibits a helical flow characteristic. The venous sinus structure facilitates the opening of the venous valve and the generation of vortices in the sinus region. However, the venous sinus structure may somewhat weaken the helical flow produced by the orthogonal venous valves. The venous sinus has a critical influence on the hemodynamics of the venous system, and in the study of venous valves, the venous sinus structure should be considered an indispensable component.