A two-stage flashing experimental unit with a controlled orifice and 80 kW electric heater system was set up. The experiments on the hydraulic and thermodynamic process of liquid flow through a submerged orifice were carried out in this device. Two types of orifice were used, one was typical orifice with flat base the other was U-bottom. The relationship between volume flow, pressure drop, gate opening and the liquid level was discussed experimentally. Moreover, a dynamic high-speed CCD photographic measurement system has been developed, which covered the techniques of high-speed vision analysis system and computer disposal system of digital images. The images of two-phase flow near the orifice were obtained. Several conclusions are obtained on the liquid level, pressure drop and temperature difference, orifice opening as well as the vapor bubble behavior.