Abstract: To enhance the steam heat transfer in collector tubes of parabolic trough solar DSG (direct steam generator) system, and to avoid too high temperature of the tube wall, numerical simulation was carried out to study the heat transfer and flow resistance characteristics in the superheat segment of DSG systems respectively with internally ribbed tube and smooth tube, so as to analyze the influence of thread height, thread pitch and Reynolds number on the heat transfer and flow resistance characteristics, following which a comprison was performed between internally ribbed tube and smooth tube. Results show that in the internally ribbed tube, steam turbulance is enhanced, heat transfer performance is improved and tube wall temperature is reduced. The Nu of internally ribbed tube is 1.6-2.4 times of the smooth tube, and the heat transfer performance increases with rising thread height and narrowing thread pitch; the drag coefficient f of internally ribbed tube is 3.1-8.7 times of the smooth tube, which also increases with rising thread height and narrowing thread pitch.