Abstract: In this paper,the ultrasonic frequency shift of in-vitro porcine liver with temperature change during the water bath heating process was analyzed and studied using the Autoregressive(AR) model,based on the discrete random medium model of biological tissues.The power spectrum density of the ultrasonic backscattered signal was computed by establishing the AR model.Statistical analysis was performed on the experimental results.The experimental results show that the mean frequencies of ultrasonic backscattered signals of the porcine liver tissues change with the temperature variation from 20 ℃ to 60 ℃.The mean frequency is(3.55 ± 0.09) MHz at 20 ℃,(3.14 ± 0.12) MHz at 30 ℃,(2.87 ± 0.11) MHz at 40 ℃,(2.79 ± 0.10) MHz at 50 ℃,and(2.72 ± 0.08) MHz at 60 ℃.These data suggest that the mean frequency decreases as the temperature increases.The mean frequency obtained using the AR model method may be used as a characteristic parameter for tissue characterization in the recognition of coagulation alterations during microwave hyperthermia,which may offer a potential pragmatic prospect for the practice of noninvasive monitoring of microwave hyperthermia.