To improve the covertness of active sonar systems, a mixed structure of Hyperbolic Modulation Frequency (HFM) signals and dolphin whistle signals is proposed for bionic active sonar communication system in this paper. With its similarity to whistle signals, HFM signals are hiding in dolphin whistle signals as covert communication signals at the transmitting terminal, which can achieve underwater covertness. At the receiving terminal, a time-scale cascade dictionary model is designed to find the optimal solution of received signal; furthermore, the two frequency variation factors of HFM signal are estimated. Simulation results show that the proposed algorithm has superior accuracy to Wavelet Transform, demonstrating that it is highly potential in bionic active sonar communication.