A quantitative liquid discharge device capable of discharging liquid in a container at preset liquid level, comprising a tubular liquid discharge pipe installed in the liquid container; the liquid discharge pipe is provided with a full-discharge liquid discharge port running radially through the wall of the liquid discharge pipe, when the liquid discharge pipe is fixedly installed in the liquid container, the full-discharge liquid discharge port is located at the lowest liquid level in the liquid container, by virtue of the full-discharge liquid discharge port, all liquid in the liquid container can be discharged; featuring that: it further comprises a tubular outer sleeve and a liquid level control rod; the liquid discharge pipe is also provided with at least one internal defined-level liquid discharge port, which is above the full-discharge liquid discharge port and runs radially through the wall of the liquid discharge pipe; the outer sleeve is cased outside of the liquid discharge pipe; the outer sleeve is provided with external defined-level liquid discharge ports, with the same number as the internal defined-level liquid discharge ports, running radially through the wall of the outer sleeve; when the wall near the bottom of the outer sleeve covers the full-discharge liquid discharge port, the external defined-level liquid discharge ports are respectively of one-to-one correspondence to the positions of the internal defined-level liquid discharge ports; the wall of the liquid level control rod is slidably placed between the wall of the liquid discharge pipe and the wall of the outer sleeve; the liquid level control rod is provided with a flow guide port running through the wall of the liquid level control rod; a sliding positioning assembly is arranged between the wall of the outer sleeve and the wall of the liquid level control rod, by means of the sliding positioning assembly, the flow guide port of the liquid level control rod can be positioned respectively between the corresponding internal defined-level liquid discharge ports and external defined-level liquid discharge ports, thus, the internal wall space of the liquid discharge pipe can be connected with the inner space of the liquid container, besides, the internal defined-level liquid discharge ports under the flow guide port are hindered by the wall of the liquid level control rod, and fail to connect with the external defined-level liquid discharge ports; moreover, by means of the sliding positioning assembly, when the flow guide port is located between a pair of internal defined-level liquid discharge port and external defined-level liquid discharge port with the same position, the outer sleeve and liquid level control rod are ganged, so that the outer sleeve can make displacement together with the liquid level control rod along their central axis. it further comprises a tubular outer sleeve and a liquid level control rod; the liquid discharge pipe is also provided with at least one internal defined-level liquid discharge port, which is above the full-discharge liquid discharge port and runs radially through the wall of the liquid discharge pipe; the outer sleeve is cased outside of the liquid discharge pipe; the outer sleeve is provided with external defined-level liquid discharge ports, with the same number as the internal defined-level liquid discharge ports, running radially through the wall of the outer sleeve; when the wall near the bottom of the outer sleeve covers the full-discharge liquid discharge port, the external defined-level liquid discharge ports are respectively of one-to-one correspondence to the positions of the internal defined-level liquid discharge ports; the wall of the liquid level control rod is slidably placed between the wall of the liquid discharge pipe and the wall of the outer sleeve; the liquid level control rod is provided with a flow guide port running through the wall of the liquid level control rod; a sliding positioning assembly is arranged between the wall of the outer sleeve and the wall of the liquid level control rod, by means of the sliding positioning assembly, the flow guide port of the liquid level control rod can be positioned respectively between the corresponding internal defined-level liquid discharge ports and external defined-level liquid discharge ports, thus, the internal wall space of the liquid discharge pipe can be connected with the inner space of the liquid container, besides, the internal defined-level liquid discharge ports under the flow guide port are hindered by the wall of the liquid level control rod, and fail to connect with the external defined-level liquid discharge ports; moreover, by means of the sliding positioning assembly, when the flow guide port is located between a pair of internal defined-level liquid discharge port and external defined-level liquid discharge port with the same position, the outer sleeve and liquid level control rod are ganged, so that the outer sleeve can make displacement together with the liquid level control rod along their central axis.