Abstract: Experiments on the density wave instability of the high pressure steam water two-phase flow in a U-type tube have been performed within the range of pressure from 4 to 10MPa, mass flux from 450 to 1200 kg/m²·s, inlet subcooling from 10 to 70℃, and heat flux from 0 to 650 kW/m². The outer diameter of the U-type tube is 20mm, and the wall thickness is 2mm. The effects of pressure, mass flux, inlet subcooling, and heat flux on the density wave instability were studied and discussed. The experimental results showed that the boiling flow system was more stable as the pressure increased. Heat flux threshold increased with an increase in mass velocity, but steam quality threshold decreased. When inlet subcooling reduced, heat flux threshold decreased. Based on the experimental results and dimensionless analysis, an empirical correlation to predict heat flux threshold was presented. The errors between the predicted threshold and the experimental data lied in a range of 15%