Medium entropy alloys have attracted much attention because of their excellent physical and chemical properties. Nano-scaled L1₂ structure ordered phase plays an important role in strengthening the mechanical properties of medium entropy alloys, and its local atomic arrangement plays a decisive role in yield strength of medium entropy alloys. In this paper, the microscopic mechanism of the precipitation process of Ni₆₀Al₂₀V₂₀ medium entropy alloy is studied by using the micro diffusion phase field dynamics model, in which the probability of atoms to occupy the lattice position is taken as a field variable to describe the configuration of atoms and the morphology of precipitates. In this model, the shape and concentration of precipitate phase, the position and appearance of new phase cannot be set in advance. Combined with the inversion algorithm, the precipitation mechanism of ordered phases of γ' (L1₂-Ni₃Al) and θ (DO₂₂-Ni₃V) is discussed by analyzing the evolution of atomic images, the change of order parameters and volume fraction. The result shows that two kinds of ordered phases are precipitated in the kinetical process of disordered phase ordering into Ni₆₀Al₂₀V₂₀ medium entropy alloys, which are of \gamma ' phase with L1₂ structure and of \theta phase with DO₂₂ structure.The two ordered phases constitute a pseudo binary system. The L1₀ phase precipitates at the same time as DO₂₂, and the L1₀ phase gradually transforms into the L1₂-γ′ phase, while the traditional Ni₇₅Al_7.5V_17.5 alloy first precipitates L1₀ phase, and then the DO₂₂ phase precipitates at the boundary of anti-phase domain of L1₂ phase. In the transition from L1₀ to L1₂, α position of fcc lattice is occupied by Ni atom, and the β position is occupied by Al atom and V atom. The congruent ordering of atoms results in the formation of θ single-phase ordered domain of DO₂₂ structure, followed by spinodal decomposition; the non-classical nucleation of L1₀ structure gradually transforms into L1₂-γ′ phase and spinodal decomposition. The interaction potential between the first-nearest-neighbor atoms of Ni-Al increases linearly with temperature, and increases gradually with the increase of long range order parameters. The incubation period of Ni₆₀Al₂₀V₂₀ medium entropy alloy lengthens with temperature increasing. This study can be applied to the design of Ni-Al-V medium entropy alloy.