Abstract: The development of deep-sea resources has attracted the attention of various countries in recent years where the mineral resources is an important part. This paper considers the internal flow in hydraulic conveying during the deep-sea mining, which is characterized by wide particle gradation and high particle volume concentration. The wide particle gradation will lead to the particle mixing and segregation, which may result in high local particle concentration. The mixing and segregation of binary particles transportation in vertical pipe is investigated based on the computational fluid dynamics-discrete element method (CFD-DEM). A virtual mass distribution function method is proposed for calculating the coarse particle volume fraction field. In addition, a weighted function method relating the particle size is given for the interpolation between the Eulerian and Lagrangian field. The two models are implanted in the open source code CFDEM based on the based on the C++ programming language. Then, the numerical method is verified by comparing the pressure drop and the minimum fluidization velocity of a fluidized bed case between the simulation results and analytical results. The study found that the mixing and segregation of binary particles will cause a gap between the front mixing area and the no-mixing area at the rear. The local particle concentration and the particle collision frequency increase also increased significantly. The particle collision stress and fluid-particle interaction stress are given, which are the ratios of unit particle collision force and unit fluid drag force to unit particle buoyant force, respectively, to explain the particle segregation mechanism. The particle mixing stage makes the particle collision stress increasing. Therefore, the moment from initial mixing to complete separation can be determined by the particle collision stress curve. In addition, the difference of fluid-particle interaction stress between the binary particle results the particle segregation because the fluid-solid interaction stress of fine particles is always greater than that of the coarse particles.