Abstract: The physical model has been built up by using parallel six head internally ribbed pipes of Φ31.8 mm×6.0 mm to do the calculation for the vertical parallel tubes with self-compensating characteristics.According to the theoretical calculation result,the vertical parallel tubes with heat flux difference between pipes have obvious self-compensating characteristics.Meanwhile,the mass velocity of water in the pipe with lower heat flux goes down first and then goes up with the increasing of heat absorption.When the steam quality reaches high level,the mass velocity in lower heat flux tube meets or exceeds the average value and the self-compensating characteristics of the system disappears.The experimental research has been carried out based on the calculation result.The experimental results show that the quality and the heat flux difference between pipes have great effect on the self compensation characteristics of the system.The mass velocity in the tube with less heat flux decreased first with the rising quality and then increased.Self-compensating characteristics of vertical parallel tubes disappears when quality gets high.The experimental results also show that when the quality is low,the increasing of the uneven heating ratio strengthens the self-compensating characteristics of the system,in contrast,while the quality is high,the increasing of the uneven heating ratio weakens it.