Using a scanning mirror to acquire large field of view is an usual scheme in infrared search and tracking system （IRST）.When the scanner rotates around optical axes,the image rotates on the detector simultaneously.Half-speed prism can correct the image rotation.In order to find appropriate prism,it is necessary to analyze the prism factors impacting on IRST.The prism parameters are obtained based on the analysis.Before assembling the prism,it must be adjusted for working normally.Test results show that the prism satisfies the demand of IRST.