Abstract: Abstract: Adding solid suspended particles into liquid is one of the effective methods to enhance heat transfer properties, and is discussed widely. The fluids of added solid particles have excellent thermal transport properties and spectral absorption properties, and can be used as a working fluid for a direct absorption solar collector to improve the collection efficiency. However, in the research on photo-thermal properties effect of solid suspended particles on heat conduction oil (HTF), the existing literature is reported less. Therefore, this paper aimed at determining clearly the influence of solid particles on solar radiation absorption properties of HTF.In this work, the photo-thermal performance tests of HTF were conducted in all-glass evacuated solar collector tubes under solar irradiation. Three experimental fluids (carbon powder-oil, copper foil-oil, and aluminum foil-oil) with different content were prepared. The spectral absorption properties and photo-thermal conversion properties were compared among the three experimental fluids through measuring the insolation temperature and solar radiation. Meanwhile, the temperature tests of water、HTF, and copper foil-oil were performed under solar irradiation. The effects of three solid particles (carbon, copper, and aluminum) and copper foil particles with different mass fraction (0.8%, 1.6%, 3.2%, and 6.4%) on the temperature rise of HTF were studied respectively. In order to evaluate the heat start-up performance of experimental fluids, the effects of solar radiation on temperature rise were also investigated by measuring the amount of solar irradiation. In addition, the mechanism of the spectral absorption enhancement and convective heat transfer enhancement of solid particles on HTF were analyzed.The research results showed that adding solid particles can improve the temperature of HTF under solar irradiation. The copper foil particles can improve the average insolation temperature by 14℃. The insolation temperature of carbon powder-oil was the highest, it was 3.3℃ higher than that of copper foil-oil and 7℃ higher than that of aluminum foil-oil in the same mass fraction. With particles content Increasing from 0.8%-3.2%, the insolation temperature of HTF increases from 98℃ to 103.8℃. However, while the mass fraction was more than 6.4%, the temperature rise was not significant. The suitable mass fraction of copper foil particles is 3%-3.2%. The temperature of experimental fluids rises with the increase of solar radiation intensity, and when the solar radiation intensity increases to 600 W/m2, the temperature increased sharply. The average temperature of carbon powder-oil and copper foil-oil were higher by 13 and 8℃ than that of aluminum foil-oil. It can be seen that the photo-thermal conversion properties of carbon powder-oil and copper foil-oil showed better than that of aluminum foil-oil in the insolation experiment.