To achieve automatic monitoring of injected water volume in separate layer water injection wells,flowmeter is installed in injection device.At present,there are many kinds of flowmeter used on the ground,which are not suitable for longterm use in the downhole,and not satisfy the space limitation inside the injection device.In order to solve the above problems,the downhole orifice flowmeter was designed.The flow measurement mode was selected by comparing the function characteristics of flowmeters used in oilfield test.Considering the factors such as flow regime,structural size constraint and pressure loss,the FLUENT software simulation was used to determine the flowmeter size.A differential pressure sensor was preferred and a protection assembly was designed.Indoor and field tests show that the flow meter is more accurate ,the measurement error is less than 5%.