To realize sustainable development, offshore wind energy has been a highly valuable solution to meet the demand for renewable energy. Accompanied with the construction of the offshore wind turbines, cable casting and the following cable trenching process in the underwater condition are necessarily required. The research on the underwater sand cutting process so far has been limited to some extent. (Miedema, 2014) has suggested the analytical solution and performed the experiments of underwater sand cutting. But all the work until then was in 2D and the description of the water influence was in the scope of statics. Thus, it is necessary to carry out the numerical simulation to describe the cutting process and the effect of the fluid by means of DEM-CFD coupling. (Chen et al., 2015) suggested a framework of modelling the underwater excavation process. The method is to describe the particle phase by Discrete Element Modeling (DEM) and the flow phase by Finite Volume Method (FVM). Starting from this method, the software package CFDEM coupling, LIGGGHTS for DEM calculation and OpenFOAM for CFD calculation, is applied. In the coupled simulation, DEM transfers the particle information to CFD. CFD solves the governing equations and updates the pressure field and velocity field. Then the fluid-solid interaction forces are calculated in CFD and transferred back to DEM. Sand particle is not spherical in reality. To describe the sand particles more accurately, a constant directional torque is added to each spherical particle to restrict its angular movement. And the sphericity of the particles is adjusted to make the sand sample in the simulation have reasonable permeability. Two sets of the numerical simulations of underwater sand cutting are conducted in this research, the simulations of 2D effect and 3D effect. The results from the simulations of 2D effect are validated with the analytical and experimental results, while the ones of 3D effect are mainly analyzed to investigate the fluid flow field and the fluid-solid interaction force. Many factors are analyzed to find out their effects on the cutting process, such as the blade geometry, the cutting layer thickness, the hydrostatic pressure, the cutting speed and the particle size. Many conclusions are found from the simulation. It turns out that the stress on the blade increases with the blade angle, the cutting layer thickness and the cutting speed. The relation between the stress and cutting layer thickness is approximately linear. And the stress has nothing to do with the hydrostatic pressure and the particle size. The features of the fluid flow field show some differences according to low, medium and high cutting speeds. The dilatation happening in the shear zone can be observed from the distribution of the fluid pressure. With a higher cutting speed, the pressure gradient in the shear zone is larger. For the fluid-solid interaction force, the effect of the cutting speed is very obvious. Besides, the dimensionless cutting forces from the simulation matches good with those from the experiments by (Miedema, 2014). From the simulation with large blade angles, a wedge between the blade and the layer cut is observed both from the velocity distinction of the particles and the stress on the blade. With the good matching in the validation work, this research confirms the reliability of DEM-CFD coupling to describe the underwater sand cutting process. Although the analysis is into depth in many aspects, further research is still needed. Using spherical particles with a constant counter torque to describe sand particles gives reasonable results. Non-spherical particle is another possible solution that is worthy to try. And the analysis about the cavitation is necessary to be performed.