Abstract: The thermo-hydraulic of multi-coupled natural circulation heat transport systems has the characteristics of flow-and-heat-transfer coupling and between-loop coupling. In this paper, theoretical and numerical methods were used to investigate the natural circulation start-up time, circulation direction and thermo-hydraulic decoupling of the multi-coupled natural circulation systems. A transient flow model for a natural circulation loop was proposed, which can predict the start-up time and flow decay of a natural circulation loop. The predictions coincide with numerical simulation. The circulation direction of a natural circulation loop with zero initial velocity was determined by the orientations of the heating and cooling surfaces. The vertical arrangement of the heating and cooling surfaces will make the system have an intrinsic circulation direction. However, when the initial velocity is reverse to the intrinsic circulation direction and exceeds a critical value, it can make the circulation direction reverse to its intrinsic direction. A simple decoupling analysis method of the thermo-hydraulic of multi-coupled natural circulation heat transport systems was proposed. It can quickly predict the thermal-hydraulic parameters of a multi-coupled natural circulation system. Both theory and calculation show that the heat transfer capacity of a natural circulation loop is proportional to 1.5 power of the temperature difference of the heating and cooling sources.