Flooding diagnosis is the key issue of PEM fuel cell. Flooding prediction through hydrogen pressure drop is investigated in this study on a two piece PEMFC stack. Pressure drop is a key parameter to diagnose water content of PEM fuel cell. The hydrogen pressure drop is more suitable for flooding prediction because it can reflect not only the content but also the water harm on PEM fuel cell. Furthermore, the ideal hydrogen pressure drop is calculated through the operation conditions. Flooding experiments are organized and a two level characteristic on hydrogen pressure drop is observed. At first, the actual hydrogen pressure drop keeps steady nearly the same as the ideal value, and the voltage is pretty high and constant. Next, the hydrogen pressure drop rises gradually because of the appearance of water droplets. Then, the hydrogen pressure drop performs the second steady level because the water droplets form a liquid film. Finally, the hydrogen pressure drop rises quickly and the voltage drops rapidly as the formation of slug flow. Combining with the visualization results of water droplet accumulation in channels and the tendency of hydrogen pressure drop and voltage, the flooding process can be divided into four continuous periods, which are proper period, humid period, transitional period and flooding period. The growth rate of the hydrogen pressure drop is defined and four groups of flooding experiments are completed, in which the effects of current, temperature, pressure and hydrogen stoichiometry can be discussed. Experiment results show that current and temperature influence little on the growth rate of two levels, while the effect of pressure and hydrogen stoichiometry is remarkable. Increasing the gas pressure can improve the ability for anti flooding in PEMFC, increasing the hydrogen stoichiometry will waste hydrogen as consideration. The hydrogen purge can take a part for a while in the flooding period to prevent the fuel cell from falling into total flooding, while it does not change the operation conditions which lead to flooding. The hydrogen pressure drop should be controlled before the transitional period. The moist section can be planned out in the first half of the humid period based on hydrogen pressure drop. In the moist section, PEM fuel cell is supposed to be far from the occurrence of flooding. Adjusting the fuel cell stack working temperature may be an effective method to realize the flooding prediction.