Abstract: A medical bi-directional in-out switchable irrigation-drainage system for intracranial surgery according to the present invention has a four-way adapter containing a knob having a rotatable interior baffle and four hollow connectors. The knob is assembled to the center of the four-way adapter. A passageway interconnects two hollow connectors next to each other and two separately isolated division passageways are formed by turning the knob. The system further has a pressure control bottle; a first connecting tube for connecting the pressure control bottle and the four-way adapter; a second connecting tube for connecting to a target tissue; and a third connecting tube for connecting to the other target tissue. The first, second and third connecting tubes are connected to different connectors of the four-way adapter respectively, and the rotatable interior baffle is actuated together with the knob.