The invention discloses an outer sleeve for a gastroscope. The whole outer sleeve is in the shape of a hollow tube. The inner diameter of the sleeve is 1.1 cm, the inner circle and the outer circle of the sleeve are not concentric, the sleeve with uneven wall thicknesses is formed, the thickness of the position where the wall is thick is 5 mm, and the thickness of the position where the wall is relatively thin is 3 mm. A washing channel is formed in the position, wherein the wall is thick, in the sleeve wall, and the washing channel extends to the bottom of the outer sleeve from the inlet end of the gastroscope. The position, close to a gastroscope inlet, of the wall where is thin is provided with a water filling pipe. The gastroscope inlet and an inlet of the washing channel are provided with silica gel plugs, and the silica gel plugs are covered with cross-shaped openings in the centers. On one hand, the washing channel in the outer sleeve can be used for washing blood, digestive juice, mucus and the like within view, and the view is kept clear; on the other hand, instruments can be delivered, the complexity and the difficulty for endoscope part treatment can be lowered, normal saline and the like are filled through the water filling pipe, the friction force between the outer sleeve and the endoscope can be lowered, the endoscope can move more smoothly and more freely, and the arranged plugs can effectively prevent injected liquid or gastric content from overflowing.