The invention discloses an elastic constant-pressure liquid supply device. A fixed support is a rectangular rigid frame; a guide rail respectively consists of two parallel channel-shaped steel girders the notches of which are opposite, is fixed in the middle of the fixed bracket and vertical to the left lateral surface and the right lateral surface of the fixed bracket; a plunger type cylinder isa cylindrical hollow cylinder with a bottom; a plunger is a rod piece with a rectangular front half and cylindrical rear half; the plunger is directly arranged in the plunger type cylinder; and a fixed support seat of the plunger type cylinder is a circular lantern ring, respectively sleeved on the outer walls of the front end and the rear end of the plunger type cylinder and connected with the outer wall of the plunger type cylinder. A connecting rod supporting seat and a guide rod supporting seat are fixed on the left side wall of the fixed bracket along a horizontal line; the left end of the connecting rod is articulated with the connecting rod supporting seat through a connecting rod fixed rotating shaft; a limiting shaft brush is clamped between two rectangular rods of a connecting rod and sleeved on the connecting rod fixed rotating shaft; and the right end of the connecting rod is articulated with the plunger through a connecting shaft. The invention has reasonable and simple structure and convenient operation, realizes constant-pressure liquid supply and can be used for soil seepage tests and earth pillar leaching tests.