Abstract: In this study the pressurizer surge line model is replaced by a vertical pipe, and visual experiments have been carried out to investigate the characteristics of air-water Counter-Current Flow Limitation (CCFL) in the vertical pipe section. The visualization experimental phenomena shows that: local-CCFL occurring at the junction between vertical pipe and upper tank limits the liquid penetration to vertical pipe; local-CCFL occurring in the vertical pipe restricts the liquid flowing out from the vertical pipe; both local-CCFL extent increases with the increasing of air flow rate. Under low gas flow condition, the water entering the vertical pipe can flow out entirely or mostly; the local-CCFL in the vertical pipe is weak, and local-CCFL at the junction between vertical pipe and upper tank dominates the overall CCFL; the overall CCFL extent in the test section increases slightly with the increasing of the upper tank water level. Under high gas flow condition, the water entering the vertical pipe is restricted partially or completely from downwards flowing out; local-CCFL in the vertical pipe is intense, dominating the overall CCFL; the effect of upper tank water level on CCFL extent disappears. New pressurizer vertical pipe CCFL model is obtained by fitting the experimental data. The present pressurizer vertical pipe air-water CCFL data overlaps essentially with our pressurizer surge line experimental data, which indicates that CCFL-U plays a dominant role on the pressurize surge line CCFL characteristics.