Although many previous studies have established the instability threshold of flooded people in terms of velocity and depth, considering that the hydrodynamic instability of people is a multiple -factor-affected phenomenon involving a complex human body-floodwater interaction, how some factors affect the hydrodynamic instability behaviour of pedestrians in floodwaters remains vague but is necessary to support flood risk management and implement tailored risk reduction stra-tegies. In this study, we adopt different physical human-body models to represent pedestrians in a quasi-natural state and test their instability thresholds in a controlled flume under six different conditions. Preliminary experimental results show that (i) the turbulent intensity of the flood-water may play a role in causing the instability of human body, and an upgraded threshold formula that accounts for this effect has been proposed; (ii) an adult's instability threshold is much higher than a child, while a female wearing a pair of heels even has a higher instability threshold than a male, particularly in deep water; (iii) the orientation of the human body also affects the instability threshold: 90 degrees yields the highest stability, 180 degrees and 45 degrees are the second and the third, while 0 degrees has the lowest value; (iv) a postural adjustment of pedestrians against the floodwater affects safety as well; (v) increasing friction tends to lead to an increased instability threshold at low depths, while friction appears to be inconsequential at moderate to large depths; (vi) a tandem arrangement of people in a crowd is more favourable to the security of a flooded child at the end of the crowd, while a parallel arrangement of the crowd reduces the safety of a child in the middle of the crowd, and a staggered arrangement yields moderate safety; (vii) the presence of a building directly in front of the flooded pedestrian markedly improves their safety, while the blocking effect due to lateral buildings plays little role. This study provides some experimental evidence and possible physical insights regarding the hydrodynamic safety of pe-destrians in urban flooding.