Abstract: The signal coherence-time is an important parameter for phase-sensitive sonar applications.The signal coherence-time can determine the bit rate and error rate for phase-coherent acoustic communications algorithms and signal gain for the phase-sensitive signal processing algorithms such as matched-field processing(MFP).In order to investigate the effects of linear internal waves on the coherence-time of MFP in the South China Sea,the internal wave spectrum is analyzed based on the oceanographic data,and the relationship between the signal coherence-time of MFP and the acoustic frequency f,the source-receiver range R,and the sound speed standard deviation(STD) δC is numerically investigated.In the simulation,the frequency is from 150 Hz to 1200 Hz,the source-receiver range is from 20 km to 32 km,and the STD δC is from 1.37 m/s to 5.0 m/s.It is found that the coherence-time can be fitted with an empirical equation satisfying a -1.3 power dependence of δC,a -1.1 power dependence of frequency in this environment, and a -0.7 and -0.5 power dependence of range with range independent and range dependent water depth respectively.