Abstract: The semi-submerged cylinder connected with a net structure is a fundamental structural component of a gravity cage. The calculation accuracy of the hydrodynamic forces directly determines the structural safety of the gravity cage in service. To effectively obtain the hydrodynamic forces acting on the semi-submerged cylinder with the net and reveal the influence of the net on the hydrodynamic forces of the cylinder, the hydrodynamic force experiments of the semi-submerged cylinder model with the net and the single semi-submerged cylinder model without the net in the steady flows and oscillating flows are carried out in the present work. The hydrodynamic forces under different steady flows and different oscillating flows are obtained, and the hydrodynamic coefficients in different flow fields are identified by the least square method. The experimental results show that the existence of the net structure will greatly enlarge the hydrodynamic forces and hydrodynamic coefficient of the semi-submerged cylinder. The influence of the net on the hydrodynamic forces of the semi-submerged cylinder cannot be ignored. Furthermore, this paper explores the influence of related parameters such as Re number, Fr number, KC number, and Stokes number on the hydrodynamic coefficients of the semi-submerged cylinder with the net. It is found that the hydrodynamic coefficients of the semi-submerged cylinder have a correlation with the Fr number, especially the added mass coefficient shows a strong dependence on the Fr number. In addition, the amplification effect degree of the net on the hydrodynamic forces and coefficients is also related to the Fr number. This work shows that using the hydrodynamic coefficients of the single semi-submerged cylinder to calculate the force of the semi-submerged cylinder with the net will cause a great risk to the safety of net cage design. In practical engineering, the influence of the net on semi- submerged cylinder should be taken into account.