Abstract: In order to quantitatively analyze the gaseous polytropic index, an expression of the gaseous polytropic index changing with the velocity of bubble wall is put forward; and the Q-X equation is modified by this expression; then the numerical calculation of acoustic cavitation by this equation is done, and compared with the experimental date. As a conclusion, it is found that different polytropic indices give greatly difference results, there is a best polytropic index for a certain experimental condition, and the Q-X equation modified by the expression of the variable gaseous polytropic index can be coincident with the date in different experimental conditions. These indicated that the expression of the gaseous polytropic index changing with the velocity of bubble wall is appropriate.