Abstract: In order to study the hydrodynamic interaction influences of the small diameter auxiliary pipelines on the main pipelines linked with the piggybacking subsea transport pipeline, a numerical simulation method for the large eddy is proposed on the basis of the characteristic-based operator-splitting finite element method(CBOS), combining the classical Smagorinsky model with the characteristic-based operator-splitting finite element method, and adopting the outlet convective boundary in the numerical simulation. The flow past around a single circular cylinder at Re=1 000 is simulated by the program, and the calculated results well agree with the results given by the other relative literatures, which has validated the efficiency of the calculation method developed by the authors of this paper in simulating the flow past around the circular cylinder. Studies are carried out on the flow past two cylinders at Re=1 000 under the conditions of different diameter ratios and different spaces in a tandem arrangement of two cylinders and the critical spacing ranges with different diameter ratios are obtained, based on the different vortex shedding forms in the flow field and the change characteristics of the mean lift coefficients and the amplitude of the drag coefficients of the large and small cylinders with different diameter ratios and spacing ratios. The wake flow in the flow field indicates that the fluid structure becomes a two-wake shedding mode instead of a single-wake shedding mode when the gap between the two cylinders is over the critical spacing. In addition, the mean lift coefficients and the amplitude of the drag coefficients of the large and small cylinders both change sharply when the gap is larger than the critical spacing. The analyses of the reasons in the increase of the mean lift coefficients and the amplitude of the drag coefficients of the cylinders are also made when the diameter ratios are equal to 0.2 and 0.4. The research results mentioned above provide a theoretical basis for the layout optimization of the piggybacking propagation subsea pipeline.