To meet the heat transfer demands in the fields of fifth-generation cellular network, artificial intelligence, and low-grade energy utilization, gas-liquid phase change is regarded as the most promising method. However, three issues of gas-liquid phase change heat transfer are desired to be solved, including high flow resistance, limited critical heat flux and heat transfer coefficient. Phase separation is found to be a general solution to solve these issues. Various technologies proposed in recent years based on phase separation were systematically summarized. Phase separation can be driven by gravity force, capillary force and heterogeneous wettability. After phase separation, the liquid or gas is exhausted out the system, reorganized inside system. It is found that phase separation can improve the thermal and hydraulic performance simultaneously. For example, cannibal fish fins were used to achieve phase separation of flow boiling, reducing drag and rising the critical heat flux exceed 1 kW/cm². The modulated heat transfer tube with a mesh membrane tube insert was proposed to eliminate the flooding of reflux condensation, converting the unstable flow into a stable flow. Moreover, the heat transfer coefficients are enhanced and pressure drops are reduced. Thus, phase separation is believed to behave great potential for thermal management applications.