The photo-thermal conversion characteristic of cavity absorber in parabolic trough solar concentrator is researched by theory, simulation and experiment. The thermal radiation loss model of cavity absorber in parabolic trough solar concentrator is established and demonstrated. Results show that while the irradiance distribution of cavity tends to be uniformity, the thermal radiation loss is reduced. The optical efficiency and standard deviation of cavity absorber at different positions are simulated by the optical design software TracePro. Results show that better optical performance can be obtained if the installed position is less than the focal length. The thermal efficiency of the cavity absorber is 35.53% with the temperature of 201.3 ℃ and the installed focal length of cavity absorber is 98.75% of system focal length which is 1200 mm parabolic trough solar concentrator by experimental verification.