Abstract: An indoor grey water purification and reuse device, system and method for public buildings is disclosed. The grey water purification and reuse device includes a grey water storage tank, a grey water treatment assembly and a pneumatic reclaimed water tank assembly which are sequentially arranged from top to bottom. The grey water storage tank is used for storing grey water from one floor above collected by a grey water collecting device. The grey water treatment assembly is used for carrying out multistage purification treatment on the grey water in the grey water storage tank. The pneumatic reclaimed water tank assembly is used for disinfecting reclaimed water subjected to multistage purification treatment and supplying water to reclaimed water reuse facilities on the current floor of the building in a constant-pressure mode.