The invention relates to a discharge system for domestic sewage of a saturation diving habitat under a large depth. A sewage discharge tank is arranged below the habitat, an electrical control cabinetcontrols an upper automatic switching valve, a lower automatic switching valve, an exchanging pipeline automatic switching valve, a gas inlet valve and a gas discharge valve, and meanwhile, the discharge system is also provided with a diving helium recycling system and a saturation diving air source system. Through the arrangement of the saturation diving air source system, the sewage discharge tank and the habitat are commonly pressurized, and through the linkage design of a toilet lid and the upper electric switching valve, risks of personal injury brought by maloperation under a high-pressure condition are avoided. The matched diving helium recycling system is adopted for recycling, then the loss of diving gas is lowered, the electrical control cabinet provides stable and reliable valve on-off control and liquid level monitoring, and stacking of harmful gas and overflow of sewage are prevented. An air purification device can assist in removing harmful and undesirable odor generatedfrom excrement, and the air environment in the habitat is improved.