Abstract: An underneath large-capacity single-gas-pump foam generating device includes a liquid replenishment mechanism and a foam generating mechanism. The liquid replenishment mechanism includes a liquid replenishment groove and a liquid replenishment bottle. The liquid replenishment bottle has a bottle opening sleeved in the liquid replenishment groove. The foam generating mechanism includes a gas supply pump unit, a gas mixing groove cavity, a gas-liquid mixing structure, a gas inlet and a vent valve structure. A lower portion of the gas mixing groove cavity is communicated with a lower portion of the liquid replenishment groove, a liquid replenishment one-way valve is arranged between the gas mixing groove cavity and the liquid replenishment groove. The gas-liquid mixing structure is provided with a liquid outlet channel, a gas outlet channel and a mixing channel, an inlet end of the liquid outlet channel extends downwardly into a bottom portion of the gas mixing groove cavity.