As an important component of offshore wind farms, submarine cables directly lay on the seabed and the magnetic field they generate may potentially have a negative impact on the eco-system, affecting the fish around, in particular. In light of this, an experiment platform was developed to expose the degree of this impact and offer a magnetic field limit. Helmholtz coils was used to generate the magnetic field and zebrafish (Brachydanio rerio) was chosen as the experiment sample. The behavior of fish under different magnetic fields was recorded by a wireless camera. Speed and trajectory analyses show that there is no significant difference between the experiment group and the control group (P > 0.05) even though the magnetic field reached 400 μT. This indicates the 100 μT magnetic field limit in the current standards may be strict. A calculation on the magnetic field distribution of typical submarine cables was carried out with finite element method and the results reveal that magnetic field falls quickly outside the cable. In most cases, it falls under 100 μT at 0.5 m from the center of the cable. Therefore, magnetic field may be an ignorable constraint in the application of submarine cables.