A seat stem adjustment device comprising: a seat stem unit having an outer tube and a seat stem which is inserted into the outer tube and linearly movable relative to the outer tube, the seat stem having a valve which has a valve rod, the valve rod having a first end adapted to control a hydraulic liquid path, a second end of the valve rod extending toward a lower end of the outer tube; a control unit connected to a bicycle frame and having a positioning unit which is fixed to the bicycle frame, a lever pivotably connected to the positioning unit, a cable having a first end extending toward the lower end of the outer tube, a second end of the cable connected to the lever, the cable being pulled when the lever is pivoted, and a leverage unit having a top member which is connected to the outer tube, a first link pivotably connected to the top member, the first link contacting the valve rod, a second link having a mediate portion pivotably connected to the top member, the second link having a first end connected to the cable and being pivoted by the cable, the second link contacting the first link which pushes the valve rod upward to be adapted to open the hydraulic liquid path when the second link is pivoted an angle, characterized by a resilient member having a first end fixed to the top member and a second end of the resilient member connected to an end of the second link, the resilient member providing a force to return the second link when the cable is released, wherein the valve rod is located in an adjustment tube, the adjustment tube has a first end extending toward the lower end of the outer tube, a first fixing portion is formed in an inside of the adjustment tube, the top member has a stepped disk on a top thereof, the stepped disk has a first stepped portion and a second stepped portion, the first stepped portion extends from a center of the second stepped portion, an outer diameter of the first stepped portion is smaller than that of the second stepped portion, a second fixing portion is formed on an outside of the first stepped portion, the first fixing portion is threadedly connected to the second fixing portion. a seat stem unit having an outer tube and a seat stem which is inserted into the outer tube and linearly movable relative to the outer tube, the seat stem having a valve which has a valve rod, the valve rod having a first end adapted to control a hydraulic liquid path, a second end of the valve rod extending toward a lower end of the outer tube; a control unit connected to a bicycle frame and having a positioning unit which is fixed to the bicycle frame, a lever pivotably connected to the positioning unit, a cable having a first end extending toward the lower end of the outer tube, a second end of the cable connected to the lever, the cable being pulled when the lever is pivoted, and a leverage unit having a top member which is connected to the outer tube, a first link pivotably connected to the top member, the first link contacting the valve rod, a second link having a mediate portion pivotably connected to the top member, the second link having a first end connected to the cable and being pivoted by the cable, the second link contacting the first link which pushes the valve rod upward to be adapted to open the hydraulic liquid path when the second link is pivoted an angle, characterized by a resilient member having a first end fixed to the top member and a second end of the resilient member connected to an end of the second link, the resilient member providing a force to return the second link when the cable is released, wherein the valve rod is located in an adjustment tube, the adjustment tube has a first end extending toward the lower end of the outer tube, a first fixing portion is formed in an inside of the adjustment tube, the top member has a stepped disk on a top thereof, the stepped disk has a first stepped portion and a second stepped portion, the first stepped portion extends from a center of the second stepped portion, an outer diameter of the first stepped portion is smaller than that of the second stepped portion, a second fixing portion is formed on an outside of the first stepped portion, the first fixing portion is threadedly connected to the second fixing portion.