The core function of non-negative pressure is defined clearly in the new version of the Industry Standard of Nation. First of all, no negative pressure: it is required to keep the pressures of municipal pipe network above the lowest service pressure. Second, difference compensation: water in tank can be used for difference compensation at peak period. Third, fully hermetic: no connection with air under compensation conditions. Forth, small flow to keep pressure: this type of water supply equipment can keep pressure of pipeline to avoid pumps and control system starting up frequently. Anyone point of the four is dispensable, and the non-negative pressure equipments with this four fuctions are the real non-negative pressure.