The invention relates to the technical field of concrete mixers for underground mines, in particular to a constant flow speed adjustable control device based on full hydraulic pressure and a control system thereof. The problem is solved that the rotation speed of an agitator tank of an existing concrete mixer for underground mines cannot be controlled to be constant. A valve block is provided witha first oil opening, a second oil opening and a third oil opening, a screw thread of the outer side of an upper valve body is in threaded connection with the valve block, the interior of the upper valve body is in threaded connection with a valve element, a limit block is in threaded connection with the bottom of the upper valve body, and the limit block is clamped in a clamping groove, corresponding to the valve element, of the upper valve body; a locking nut is in threaded connection with the valve element, an adjusting hand wheel is in interference fit with the valve element, a pull rod seal is mounted in a sealing groove of the valve element, and a valve element casing is clamped at the lower end of the valve element; a spring inner casing is embedded in the end of the valve element casing, a spring sleeves the spring inner casing, and a sliding casing sleeves the outer side of the spring; a check ring is mounted in a clamping groove of a lower valve body, and a first fixed seal and a second fixed seal are mounted at a sealing groove of the lower valve body.