To design signal of low interception active sonar exactly, on the basis of derive sonar interception probability factor equation, signal characteristics are analyzed from the view of low frequency, large BT product, compound code, random or nonlinear, cross section of time-frequency and energy ratio. The results demonstrate that main means of enhancing low probability of interception performance of active sonar are that signals with characteristics of low frequencies, wide brands, coded pulse chain and low power transmission ratio.