With development of anti missile weapons, the interception area has extended from re-entry phase to the whole unpowered phase. Whether the intercept missile is properly launched in time will directly affect effectiveness of unpowered phase interception. Firstly, the intercepting time window was depicted in math model. Then, the ballistic models of the TBM's unpowered phase and the intercept missile were established respectively. According to the specialty of both sides in the interception, the judgment term and algorithm for the intercepting time window were given. Finally, through an example of the interception of different range TBM for one certain type interception missile, the corresponding intercepting time window was calculated. The result shows that the range of TBM is closely related to the intercepting time window when the capability indexes of intercept missile are fixed.