Ejecta production from the metal surface under shock-loading is currently a focused issue both at home and abroad. However, the traditional experimental techniques, such as piezoelectric pin, only diagnose the ejected data for low-density ejecta but not for high-density ones, giving a poor understanding of this process. Particularly, when ejecta production increases significantly as the loaded metal melts on release or shock, the measurement carried out by the traditional piezoelectric pin becomes worse, and brings further missing knowledge in the ejecta evolution.In this paper, an Asay-F window designed earlier by the authors based on the traditional Asay-window, is employed to investigate the formation process of the ejecta from the melted Sn metal. As indicated by previous experimental findings on shocked Pb sample, the Asay-F window is a reliable and effective tool for measuring the high-density ejecta by comparing the result with those of the piezoelectric pin. The interface velocity within the Asay-F window measured by Doppler pin system, is obtained. On the basis of momentum conservation condition, the physical quantities of ejecta, such as accumulative areal mass, volume density and velocity, are derived from the interface velocity. By analyzing the experimental data diagnosed by the Asay-F window, which is placed at different offsets from the free surface of Sn sample, the expansion evolution of the ejecta is obtained. Through transforming the dynamic volume density to the static one, the picture of the ejecta density distribution changes with the spatial distance at a specific moment, which is explicitly displayed. It is found that the ejecta density distributions gained from the different offsets at the uniform moment are consistent. As a consequence, the self-similar expansion evolution of the ejecta is experimentally confirmed, which successfully avoids the unclear understanding of this process if only examined by the piezoelectric pin. This experiment may lay the foundation of the formation of the ejecta production for the metal sample subjected to high pressure loading.