In this paper, radiation characteristics of target /b ackground and atmospheric transmittance are analysed. By analysing the result of the test, infrared focal plane array sensor working within 3～5μm wave band is considered to be appropriate for detecting and tracking conventional fast movin g object. Based on this, the detecting distance for 122mm rocket using infrared focal plane array sensor within 3～5μm wave band and the precision of two infra red measuring system intersecting measurement are discussed.