The present application relates to an indoor grey water purification and reuse device for a public building, a system, and a method, pertaining to the field of urban water conservation, and solving the problem in which cascaded cycling and use of indoor water supply and automatic control for stable operations cannot be realized in public buildings. The grey water purification and reuse device comprises a grey water storage tank, a grey water treatment assembly, and a pneumatic water tank assembly for reclaimed water which are sequentially arranged from top to bottom. The grey water storage tank is used to store grey water of a previous floor collected by a grey water collection device. The grey water treatment assembly is used to perform a multi-stage purification treatment on the grey water in the grey water storage tank. The pneumatic water tank assembly for reclaimed water is used to disinfect the reclaimed water after the multi-stage purification treatment is performed, and to supply, in a constant pressure mode, water to a reclaimed water reuse facility of a current floor of the building. The present application realizes collection, treatment, and reuse of indoor grey water in public buildings, and enables the indoor grey water to replace tap water for toilet flushing, thereby significantly reducing tap water usage in public buildings, and improving water-saving efficiency for cities.