The invention provides a flow controlling device of a gate type magnesium alloy cast rolling machine front box, and belongs to the field of magnesium alloy cast rolling. The device mainly comprises atransmission device, a transmission locking mechanism, a detachable gate flow controlling valve and a sealing device, wherein the transmission device is connected with a gate of the detachable gate flow controlling valve to drive the gate to move up and down, the transmission device is connected with the transmission locking mechanism in series and is arranged on the upper side of the outer surface of the front box, and the sealing device is arranged on a gate connecting rod of the detachable gate flow controlling valve in a sleeved mode. According to the device, the gate is driven by the transmission device to move up and down, the transmission locking mechanism is connected with the transmission device in series, so that accurate control to the flow of a valve port is ensured; the improved detachable gate flow controlling valve solves the sticking problem generated after the valve is thermally deformed, the replacement is convenient, so that the labor intensity of workers is reduced,and the production efficiency is improved; and according to the overall structure of the flow controlling device, a good way is provided for a simple, safe and reliable sealing mode, and under a high-temperature environment, the device has outstanding advantages in the aspects of flow controlling precision and overall sealing performance of the structure.