Abstract: An optical system of a laser Doppler velocimeter based on dual-beam mode was designed for flow velocity measurement in field, and error analysis was discussed with experimental results performed. In order to create interference fringe, the beam of a fiber-coupled laser diode was split by a polarization maintaining fiber coupler, then both the beams behind the fiber coupler were collimated by fiber collimator and focused by an aspheric lens. In order to detect Doppler signal, a collecting lens and pinhole was used to project the useful scattering light onto an avalanche photo diode. Resolution of the built optical system was 0.046mm3, and measurement error was less than 0.59% in theory. Because of its advantages such as small size, integration and convenience, the system is suitable for field measurement.