Fluids play an important role in subduction factories. Petrological studies usually focus on “immobile fluid” in minerals, while research on free flowing fluid in subduction zones, especially the process and effect of the immiscible fluid evolution and subsequent separation, is relatively lacking. Fluid immiscibility and evolution are common in nature, which has been verified by natural sample studies, high P T experiments, and theoretical calculations of the equation of state for fluids. The actual fluid system in the subduction zone is a multivariate, open and complex system. As the fluid migrates and evolves, the fluid and fluid rock system become more diverse. Therefore, fluid immiscibility and evolution have the characteristics of universality, complexity and diversity. Geophysical evidence indicates that fluids in the subduction zone are concentrated in the subducting slab near the trench, near the interface between the subducting slab and the overlying mantle wedge, and inside the overlying mantle wedge. These single phase or multi phase fluids can migrate through a variety of paths, mechanisms and transport modes. On a large scale, the fluids flow updip under a sealed plate interface between the subducting slab and the overlying mantle wedge. In some locations (such as deformations or fractures/faults), the seal breaks, and fluids escape upward through vents into the mantle wedge. Fluids entrapped by rocks or migrating downward at bends in the subducting slab are not excluded. Locally, fluid migration is governed by different lithologies and structural interfaces with anisotropic permeability. By combining the possible paths of fluid migration in the subduction zone, the fluid P T X phase diagram in the high P T range, and the thermal structural model of the subduction zone, we built and discussed an ideal theoretical model for the immiscibility evolution of fluid in simple binary system during migration in the forearc of the subduction zone. On the one hand, the fluids in subduction zones with different thermal structures have different circulation and evolution characteristics. Compared with hot subduction zones, cold subduction zones have a deeper and wider fluid immiscible space. On the other hand, in the same subduction zone, different system fluids have different spatial ranges where immiscibility can occur (for the binary system, H 2O CO 2 < H 2O CH 4 < H 2O N 2 < H 2O H 2). Different positions and different migration paths of fluids in the subduction zone also control the evolution of fluids. In addition, the gradual cooling of the subducting thermal structure with long term evolution, and the cooling of the Earth on longer time scales expand the spatial extent of the immiscibility of fluids in the Earths interior, which may promote the sequential or simultaneous release of these fluids over the long term and further have a significant impact on the Earths internal and surface environments. The evolution process of fluid immiscibility, separation, selective migration and enrichment directly affects petrologic research, the formation of mineral deposits, and changes in the environment and climate, and is associated with a variety of geological phenomena and processes (such as cold springs, hot springs, mud volcanoes near tectonic margins and the formation of quartz and carbonate veins commonly found in nature). The impact of fluid immiscibility on the physical and chemical processes of the Earth’s interior may be greatly underestimated. Therefore, the purpose of this paper is to hopefully draw more geologists attention to the problem of fluid immiscibility.