Abstract: Abstract: Parabolic trough solar collector (PTC) is one of the most proven technologies in medium and high temperature solar thermal utilization field. The parabolic trough vacuum tube receiver, also called the heat collector element (HCE), plays a crucial role in the parabolic trough collector system. HCE performance directly affects the performance of solar energy heat utilization system. It is effective to improve the HCE performance by decreasing heat loss in annular space between the absorber and glass envelope. And right annulus gap size and pressure in annular space are important to decrease heat loss in annular space and improve the overall efficiency of the PTC. In order to optimize the annular space by using theoretical methods, a three-dimensional flow and heat transfer model is proposed. The cross-section profile of the solar energy flux on the outer surface of the absorber tube obtained by the SolTrace software is set as the thermal boundary condition in the model. To verify the validation of the model established in this paper, the thermal efficiency of PTC is calculated and compared with the experimental data obtained by literature. Satisfactory agreement is found and the average deviation between the simulation and the experiment is about 4.91%. On the basis of reliable results numerically simulated, the effects of the key parameters such as annulus gap size, pressure and filling gases on the heat-transfer characteristics of the receiver are numerically investigated under different conditions. Numerical simulation results indicate that temperature and radiation intensity distribution in annular space are not uniform because of the effects of non-uniform heat flux on the absorber tube wall. And thermal efficiency of PTC is significantly affected by annulus gap size. Average temperature of absorber tube's outer wall and glass envelope's inner and outer wall decreases with the increase of annular gap size. And heat loss in annular space decreases with the decrease of average temperature of glass envelope wall. Annular gas size below 20 mm can significantly affect the heat loss in annular space. Size above 20 mm, however, has slight influence on the heat loss in annular space. The HCE performance as a function of the pressure in the annular space is also studied. Average temperature of absorber tube's outer wall decreases and average temperature of glass envelope's inner and outer wall increases with the increase of annulus pressure. Annular pressures between 0.0001 and 0.01 Pa have slight influence on the HCE performance. But at annular pressure above 0.01 Pa, heat loss becomes a problem for HCE. Currently, the HCEs are manufactured with the annulus space under a vacuum of 0.01 Pa. The heat-transfer characteristics of the annular space are influenced by the type of gases in the annulus space. Average temperature of glass tube inner wall, as a function of annular pressure, is compared among air, hydrogen, helium, nitrogen and argon filled in the annulus space. The inert gas with a low thermal coefficient, i.e. argon, results in the lowest average temperature of glass envelope inner wall and the best HCE performance. However, air provides only a slight improvement on HCE performance. Hydrogen increases average temperature of glass tube inner wall significantly. The encouraging results in this paper will provide a fundamental reference for researching heat-transfer characteristics in annular space and optimization design of the PTC.