The massive attachment of fouling organisms will clog the net of fish cages, reduce the water exchange within the cages, and increase the structural load and deformation, which will lead to an increased risk of damage to the cage under severe sea conditions, and may cause significant economic losses. This study conducted a field test by deploying nets in marine cage aquaculture areas, obtaining nets with varying levels of biofouling (Sn=0.213-0.442) to study the effects of fouling organisms on the hydrodynamic characteristics of fish cage nets. The composition of the fouling organisms was analyzed, and flume tests were conducted to explore the hydrodynamic characteristics of hydroid-fouled nets for different flow velocities (u=0.2-0.5 m/s) and angles of attack (θ=0°-90°). The results indicated that attachment of fouling organisms initially increased and then decreased with the duration of immersion time, with the maximum attachment occurring at a depth of 4.5 meters during the fourth week. There was a positive correlation between the wet weight of the net and the degree of attachment. Notably, the attachment of fouling organisms like hydroids significantly alters the hydrodynamic characteristics of the nets. The net with the most severe fouling (S_n=0.442) exhibited a 6.09-fold increase in maximum drag force and a 5.99-fold increase in maximum lift force compared to a clean net (S_n=0.146). The influence of hydroid-fouled nets on their hydrodynamic coefficients under varying angles of attack exhibits notable differences, the drag coefficient could increase by as much as 2.1 times, and correspondingly, the lift coefficient underwent a 2.0-fold enhancement, in comparison to clean nets. At an angle of attack of 90°, the relationship between the drag coefficient (C_d) and solidity ratio (S_n) of the hydroid-fouled nets was well-fitted by the equation C_d =0.42+8.98S_n–7.78S_n2 (R2=0.803, S_n=0.145–0.442). Our research showed fouling organisms, like hydroids, significantly affected the hydrodynamic loads on fish cage nets, worsening stress distribution, and increasing the risk of damage to the net. Therefore, in the design and safety assessment of cages, it was essential to comprehensively consider the impact of changes in net drag force and lift force on the structural integrity of the fish cage. Moreover, fouling organisms should be removed from the nets promptly to ensure structural integrity and safety during the production process of cage aquaculture. This study provides valuable insights for the design and optimization of fish cages, as well as for the scheduling of net cleaning practices. By addressing these aspects, we can enhance the durability of cage net systems, ultimately contributing to more sustainable and efficient aquaculture.