Trawling energy consumption is significantly affected by the drag on the trawl, which is determined by a series of design and operational variables. In this study, a fluid-net interaction model of a trawl system is developed to understand the hydrodynamic behavior of trawl. The k-omega shear stress turbulent (SST) model is applied to simulate the flow field through the net cage, and the large deformation nonlinear structural (LDNS) model is adopted to conduct the structural analysis of the flexible net cage. To validate the numerical model, the drag forces on the single circle cylinder and the net panel are calculated and compared with the experimental data. The results indicate that the numerical model can be applied to calculate the drag force on the net panel. Then, the numerical model is adopted to calculate the drag force on the trawl, and the effect of twine diameter on the drag force acting on the trawl net is analyzed. In addition, an existing equation for analyzing the drag force on trawl net was expanded to consider the effect of twine diameter using this numerical model.