Square channel is widely used in the conceptual design of water-cooled blanket of fusion reactor for cooling and providing appropriate inner temperature field for tritium breeding. Blanket is one of the most important safety components, and the thermal-hydraulic characteristics of blanket directly determine the heat transfer efficiency and safe operation of fusion reactor. Under accident conditions, the natural circulation phenomenon occurs without any mechanical devices intervention when the field forces acting on the fluid produce density gradients able to induce natural convection, which are the main heat transfer mechanisms and the important measures to mitigate the consequence of the reactor accident. For square channel in blanket, the experimental study of natural circulation heat transfer has been conducted. It can be found that natural circulation mass flow is not an independent parameter, which increases with the increase in the heat flux and with the decrease in system pressure within the experiment scope. A correlation that can correlate the experimental data with the maximum deviation of 13% is established to predict natural circulation mass flow. Simultaneously, natural circulation heat transfer is strongly affected by system pressure and heat flux. And new correlations are developed on the foundation of experimental data to predict the heat transfer coefficients of natural circulation under different conditions. Comparing the experimental results with the results of forced circulation, it can be found that the heat transfer intensity of natural circulation is lower than that of forced circulation.