This study was conducted to investigate the relationships between parameters affecting internal quality of watermelon and its acoustic characteristics. The measuring and analyzing system was established to study the acoustic characteristics of watermelon. Algorithms for analyzing sound signals were developed. Sound signals which was detected with the microphone were filtered, and their spectrum was computed by means of the Fast Fourier Transform. As watermelon changed from the unripe stage to ripe, acoustic waves in time domain became complicated, and several components appeared in frequency domain. The correlationship was investigated between some parameters affecting internal quality of the watermelon and several peak frequencies. Results indicated that weight, density and sugar content had high correlations with several frequencies(the first peak frequency, the second peak frequency, and the third peak frequency). And the sugar content and the volume of the watermelon were highly correlated with the third peak frequency.