Abstract: In this paper the results of an experiment concerning internal wave and acoustic propagation fluctuation in lake water are presented. The thermocline occurs in summer time in the shallow water region of the inland lake, the temperature difference is about 9℃ and the thickness of transition layer is about 30 m. Through many measurements, it is detected that there are internal waves at the depth 14-45 m of the inland lake. The change of the temperature recorded at the fixed depth in the thermocline is as large as 2.5℃. The amplitude of the internal waves is as large as 5 m, with apparent frequency components of 8-16 hours.The nonsychronism of the time dependence of the isothermal curves indicates that the internal waves have multimode structure in vertical direction. The internal wave displacement spectrum decreases with increasing frequency in the rate about w^-2. The rms deviation of the internal wave displacement is 0.61 m. The multipath structure will occur when an impulsive acoustic signal at the frequency of 4.48 kHz propagates in the neighbouring region. The fluctuations of direct sound are influenced by internal waves. When impulsive width is wider than 1 ms, (with r=495 m, H=70 m), direct sound and reverse sound in thermocline layer will mix up with each other, which causes the fluctuation of propagating acoustic signals, with the amplitude of ±1.4 dB. The decreasing rate of log sound ampltitude spectrum is w^-1. The rms deviations about 0.1-0.3 dB of those fluctuation are not so large as those in shallow water. What should not be neglected is the time extension of receiving acoustic signals. It will make certain difficulties for underwater location.