The utility model provides a flow monitoring device mainly by sharp -crested weir formula flow measurement spare, have the efflux and drive the drain hole pipe of sediment accumulation effect, level sensor and data processing unit and form, the bottom at flow measurement spare is installed to the drain hole pipe, be equipped with the vertical spoiler of preventing below the formula flow measurement spare delivery port minimum of weir and between the drain hole pipe to restrain the efflux and clear away the silt process to the disturbance of sharp -crested weir formula flow measurement in -process to perpendicular to water flow direction, formula flow measurement spare delivery port peak central line bilateral symmetry is equipped with the guide plate on the weir to the requirement of sharp -crested weir formula flow measurement in -process to rivers lateral stability keep, the liquid level measurement sensor is installed on being fixed in the support of guide plate, and sensor output signal transmits the data processing unit to through the wire, according to same level sensor testing result, can be simultaneously calculate the flow that flows past sharp -crested weir formula flow measurement spare and the flow through the drain hole pipe, confirm total discharge in the drain pipe of tunnel center.