There have been numerous urban flood disasters in recent years, causing a considerable loss of human life. An improved criterion in the form of incipient velocity has been proposed for human stability in floodwaters, and it is specifically for deep waters where toppling instability generally occurs, and it can account for the posture of leaning forward of a human body in floodwater and the process of pivoting around heels at instability. Moreover, the previous equation of buoyancy force was modified using the parameters of human body structure for a typical American or European in ergonomics. Two parameters in the formula of incipient velocity were calibrated using existing experimental data based on real human subjects, with the improved stability criterion curves being presented for children and adults in floodwaters. Finally, the proposed formula was validated in detail against existing laboratory measurements, and the simple method was given to evaluate the flood hazard degrees for children and adults, based on the calculated incipient velocities at toppling instability and the corresponding incoming flow velocities.