The invention relates to a building water saving system. The water discharge pipelines of water using utensils of each family are connected to water inlets of water storage tanks of flush toilets; thewater inlet of each water storage tank is lower than the water outlet of each water storage tank; the water outlet of each water storage tank is connected to a water storage tank in the next floor, the water outlet of the water storage tank in the ground floor is connected to a water storage tank (A) in the bottom of the building; a water pump is arranged in the water storage tank (A) in the bottom of the building; the water pump is connected to a water storage tank (B) arranged in a higher position of the building through a pipeline; the water storage tank (B) is connected to water dischargepipelines of water using utensils of each family through pipelines and then connected to the water storage tanks of flush toilets; and the drain pipelines of flush toilets in a plurality of floors are connected to a digestion tank through a header pipeline. By improving the water discharge pipelines and flush toilets of each family, the sewage can be utilized in cycle, the water resource waste isgreatly reduced, energy and water are saved, and the system benefits the nation and the people.