A self-starting negative pressure drainage system for draining groundwater in a slope, includes: a declined borehole, a pipe boot, a permeable pipe and a drain pipe; wherein the declined borehole is divided into a permeable drilling section and a sealed grouting drilling section; wherein the permeable drilling section is on a lower portion of the declined borehole, and the sealed grouting drilling section is on an upper portion of the declined borehole; a water stop ring made of water-expanding rubber is provided between the permeable drilling section and the sealed grouting drilling section; the permeable drilling section comprises the permeable pipe; a top of the permeable pipe contacts the water-expanding rubber; a cavity is formed in the permeable pipe, an inlet of the drain pipe passes through the water stop ring made of water-expanding rubber and is inserted into the permeable pipe.