An optical delay line system for NIAOT Prototype long baseline stellar optical interferometer is being developed. The delay line system consists of optics part, machine part and control part. The optics part is a cat’s-eye system which includes a paraboloidal mirror and a flat mirror. The flat mirror is placed at the focus, and is driven by a piezoelectric actuator for real-time compensation of the tracking error. The defocus of the

flat mirror caused by this compensating is considered in optical design; and that the aberration of the optical system design and the manufacture precision of optical components should not cause the decline in visibility of the fringe is analyzed, also. The machinel part includes precision rails and delay line carriage. The rails require high stability and parallelism. The cart should be quakeproof when it is moved continuously in the observation process, so the rolling friction drive mode is selected as the suitable link method between the carriage and the rails. The control part includes delay line carriage device control and laser metrology system device control. During an observation, an astrometric model provides a demand cart position and velocity to control computer, the control computer send them to the device controllers. The metrology system produces tracking error fed back to the cart device controller via the control system. This feedback servo loop controls the tracking error.