<p>To reveal the gas-water two phase flow characteristic in the anode porous transport layers (PTL) of Proton Exchange Membrane (PEM) electrolyzer, both experimental and numerical (twin numerical model) studies were carried out in this work. An optical visualization test setup was developed, high-speed camera was used for flow visualization, pressure sensor and flowmeter were used to quantify the pressure drop and mass flow rate characteristics of gas-liquid two-phase flows, and the pressure drop and flow regime in the anode free flow channel as well as the PTL were reveal under different gas-to-water ratio (GWR), incline angle, porous size and pores configuration. Besides, a twin numerical model was also developed, and the flow characteristics of gas-liquid two-phase flow were further revealed by simulation. Moreover, the bubble coverage <i>α</i> in the free flow channel and the water proportion in the catalytic interface were quantified, and the threshold GWR under different operation conditions were also determined, which provides insight for water management of PEM electrolyzer.</p>