An optical director design of the infrared search and track （IRST） system is proposed. After analyzing characters of the IRST system and its optical director,the compositions and functions of the optical director of the IRST system are described.IRST system requires a large search field, a short search period and response time. In order to get these effects, this scheme scans the search field with mirror polygon combining overturn mirror, by this means,it is able to meet all demands of an IRST system. With regard to the control system,different electromotors, encoders and control methods are applied according to different requirements for position and speed of each component to reduce volume and cost at the same time to meet the precision demand.Testified by experiments, this infrared optical director has advanced technology and rational scheme and can well meet all demands of an IRST system. The experiments of searching targets have excellent results. An innovation of this paper is that it resolves effectively the conflict between the demand for the large-field and rapid search and the demand for the small volume and the low cost.