Abstract: In the flow field measurement in conical tube with LDV (such as straight cone section, gradually expanding section of hydro turbine tail pipe), due to large outer wall curvature and sloped inner wall of conical tube and light refraction in the tube, the axial flow velocity cannot be measured by LDV. So, a measurement scheme for flow field in conical tube is put forward. Through theoretical derivation, this paper analyzes LDV measurement light route in the conical tube, and demonstrates the measurement scheme feasibility. We deduce the linear relation of the measurement point and LDV light source and the variation range of SDV light source location, which can provide an effective and feasible scheme for the similar measurement.