A horizontal annular temperature field has been designed on the basis of similarity principle to investigate the formation mechanism of core-shell microstructures of monotectic alloys during free fall. The phase separation process of transparent succino-dinitrile-52.6mol%H2O monotectic solution was observed directly. The H2O phase separated from the parent liquid at first, then moved to the center of the sample, and at last occupied the core part. The moving velocities of H2O droplets were measured according to the photographic pictures, which are in good agreement with the theoretical prediction of Marangoni migration velocities. The experiment reproduced the liquid phase separation process of monotectic alloy under microgravity condition.