Slag fly ash based geopolymeric concrete（SFGC） was prepared. Compressive strength and longitudinal wave velocity of SFGC under various temperatures and different cooling methods were investigated through compressive strength and ultrasonic test. The acoustic signals were analyzed by using wavelet packet transformation（WPT). The results indicate that acoustic spectral characteristics obtained by wavelet packet analysis is more accurate and sensitive under different working conditions. With the raise of temperature and progress of damage, compressive strength and longitudinal wave velocity decrease constantly, power spectrum and signal energy in each sub band change significantly, and the frequency spectrum and energy spectrum center shift to the lower frequency side. The mechanical and acoustic characteristics of SFGC cooled by spraying water change greatly compared with that cooled naturally. The WPT based spectrogram parameters are sensitive to the thermal damage so that they can be used to distinguish the working condition and damage degree of SFGC.