The display device may include a display unit and a support unit. The display unit is used for displaying patient-end parameter information. The support unit comprises an adapter cover and a rotating unit connected onto the adapter cover. The display unit is fastened and connected onto the adapter cover in a way that allows rotation relative to the rotating unit. The monitor comprises: a master control unit and the display unit, where the master control unit is connected to a patient end via at least one cable, and the display unit is detachably connected to the master control unit. With the implementation of the monitor, the display device thereof, and the monitoring system, the display unit rotates and switches while the position of the connecting cable of the master control unit remains unchanged; corresponding marker display areas and alarm light display areas are lit up after rotating and switching in order to taking into consideration habits of a user.