In underwater acoustic communication systems, channel characteristics are mainly affected by changes in space and time. The changes embody the random fluctuations of the seabed and sea surface, and the effects of refraction and scattering caused by seawater layered media on the sound field. As the channel is time-varying and space-varying, the rapid fluctuation of communication signals, which is shown in the frequency selective fading in frequency-domain and the signal waveform distortion in the time domain, leads to a bad effect on the performance of the underwater acoustic communication system. The techniques to compensate or offset the effects of deep fading such as coding error correction and spatial diversity are widely used in underwater acoustic communication systems, which may cause a significant waste of limited communication efficiency. This paper analyzes the spatiotemporal fluctuation characteristics of both signal field and noise field, and summarizes the temporal and spatial variation rules. The effects of signal spatiotemporal fluctuation on communication systems are significantly reduced by reasonably selecting the communication signal parameters, equipment deployment depth and horizontal distance, which will guide the parameter configuration and network protocol optimization of communication systems.