Firstlythe definition of detectability was presentedwhich is a new physical quantifier used to describe the difficulty level for detecting a target.Analysis was given to the influence of target flight altitude and velocity on its detectability in infrared bandand the regularity of detectability changing was obtained based on a mount of simulating calculations.The analysis result shows that:1) Under normal weather conditionthe active detecting range will increase by 5 km for the same infrared detector when the speed of target improves by 0.5 Mach;2) The detectability of a target will increase with the increasing of its flying height when the height is under 8 kilometersdecrease with the height when it is beyond 10 kmand reach the max at the height of 8 to 10 km.