Abstract: For countries where coal is the main energy source,thermal power units,especially coal-fired power units,will continue to operate with low load or deep peak adjustment in the next few years. In the process of depth peak adjustment,the load of the unit deviates from the design condition,which may lead to flow instability. In this paper,the formation mechanism and influencing factors of the flow instability of ultra-supercritical units was analyzed and studied in order to provide guidance for the design of heat transfer equipment and safe variable load operation and the research methods of flow instability in ultra supercritical units were analyzed. According to occurrence characteristics,flow instability can be divided into static instability and dynamic instability. In load changing process of ultra-supercritical boiler flow system,there are flow instabilities such as density wave,pressure drop and thermal flow instability,which will have a serious impact on the normal operation of the system. The main influencing factors of flow instability include heat load distribution,pipeline structure and system flow parameters. Due to the development of analysis and calculation tools,the occurrence conditions and change rules of flow instability can be predicted accurately. A large number of experiments and numerical studies show that the lower the heat flux density is,the higher the system pressure is,the larger the inlet throttle coefficient is,and the smaller the outlet throttle coefficient is,and the more stable the system is. From the perspective of pipeline structure,the shorter the heating length is and the larger the inner diameter is,the more stable the system is,and the system with cross-connection is more stable than the system without cross-connection and the single-channel system. There are two methods to study the instability of ultra-supercritical water flow: experimental test and numerical simulation. The advantage of the experimental method is that it can focus on the actual physical system as the research object and provide valuable reference for the corresponding numerical simulation. Considering that the experimental system of water flow instability under ultra-supercritical pressure and temperature is extremely complex and requires a large amount of cost,numerical simulation has become an important research method. Numerical simulation can learn from mature two-phase boiling research results,which can easily analyze the influence law of various parameters on flow instability. For the numerical simulation of the flow instability of supercritical fluid system,the analysis methods can be divided into frequency domain method and time domain method. The disadvantage of the frequency-domain analysis method is that it can't solve the nonlinear problem well. The solution to the disappearance of the nonlinear effect of frequency-domain analysis method is to determine the amplitude of the limit cycle through the Hopf bifurcation technology. To analyse the flow instability of ultra supercritical fluid,the time domain method is most commonly used in the analysis for period of oscillation and chaotic nonlinear effect. With a series of dimensionless number,both dynamic changes and the flow instability boundary of subcritical and supercritical fluid can be effectively described.