In this study, the parameters of Gassmann equation based on fluid replacement theory are studied by measuring the acoustic velocity during the evaporation process of volcanic rocks in Nanpu area. The experimen-tal data show that with the decrease of porosity of tight volcanic rock, the acoustic velocity difference between dry and wet rock samples increases, which is conducive for the identification of gas bearing reservoirs with acoustic log data. The fluid bulk modulus distribution of volcanic rocks in the study area conforms to Brie mod-el, and the value of empirical coefficient e is related to lithology. The experimental results show that there is a linear relationship between the P-wave transit time of dry and wet rock samples. Using porosity to calculate the acoustic transit time of saturated rock samples, and taking it into the experimental formula, we can get the P-wave transit time and bulk modulus of dry rock samples. According to the bulk modulus of mixed fluid, dry rock and rock matrix determined by experiments, the saturation of volcanic reservoir in Nanpu area is calculated by Gassmann equation, which is in good contrast with the conclusion of gas test. This study provides an experi-mental basis for quantitative evaluation of volcanic gas reservoirs using seismic and acoustic logging data.