Abstract.A novel paddlewheel flow sensor has been developed which based on a hydrophobic fibre-optic pick-up is designed in this paper, it presents the theory, operation principle, design of the sensor. The hydrophobic fiber optic head is the sensing part, which can provide two different reflective signals output by rotation of rotor, light pulse frequency is proportional to the flow-rate. Signal acquisition and processing is realized based on LabVIEW, the information of signal waveform and flow velocity in a pulse time can be processed and displayed on the front panel real time. The flow sensor provides a completely passive measuring system for gas and liquid flows, it is rather cheap and inherently safe. The detector unit can provide either a pulse output or convert the signal into flow units. Preliminary experimental testing on the flow sensor has been carried out, the results confirmed that the design is feasible.