The high sensitivity and reliability of the biosonar have attracted many bionic scientists' attention. However, there is no convincing physical model to explain the reasons of the superior performance of biosonar. The main reason is that the neuron coding of the nervous system is still uncertain. Based on the physiological structure of the bat's auditory nervous system, a probable explanation is proposed to discuss the Doppler signal process with the principle of circle maps and symbolic dynamics. Through the computer simulation, the rationality of this method is proved. For the instability of the nervous system, using symbolic dynamics to analye the mechanism of the neural information processing has high sensitivity and robustness. It is expected that the research of this new explanation will be able to promote the understanding of the biosonar signal processing and its applications.