The invention provides a water saving device to recycle domestic wastewater automatically. The device utilizes the surplus pressure generated when urban tap water pipe net supplies water to families to enhance potential energy of the domestic wastewater, recycles domestic wastewater and automatically supplies to toilet according to the hydraulic principle. The device comprises a water bag, a filter, a water storage tank, a connecting rod, a reversing valve, a check valve, a box shell, etc. The device is driven by surplus pressure of the tap water, recycles the domestic waste water to flush toilet, needs no other power to be added, saves domestic water using, can replace water treatment device of the community and has great importance on reasonable water using and relieving the serious status of water shortage in China.