Phase change thermal storage used metal as phase change material (PCM) is an important mode of solar thermal energy storage. In this paper, the heat charging processes of solar heating units were simulated under three kinds of heating flux 100,150 and 200kW/m2 at the bottom face respectively, while the thickness of heat receiving layer at the bottom was in 5, 10 and 15mm. Al-Si alloy was selected as PCM used in the cylindrical body of the units which were in the size of φ1000×1000mm. The change of temperature and solid-liquid phase change interface of Al-Si alloy were analyzed to find out the suitable absorber thickness of thermal energy storage units which can run safety under the condition of temperature 700~900K and heat flux 100~200kW/m2, such as the application of solar thermal energy storage unit in high temperature solar thermal power stations. In the last a test system was built up and the experimental result was close to the simulation value of a unit in the size of φ300×1000×10mm.