Abstract: Biologic layered tissue is the most common acoustic channel in high intensity focused ultrasound (HIFU) treatment. The effect of biological tissue on acoustic focal region is studied to provide a theoretical basis for the safety and reliability of HIFU treatment. Based on Westervelt equation, the finite element algorithms in frequency domain and time domain are used to simulate the focal region changes when ultrasound penetrates layered tissue during HIFU treatment. And experiments are carried out with fresh pork tissue in vitro. The results show that:The differences in acoustic characteristics and structural heterogeneity of biological tissues change the acoustic focal position of HIFU, especially in the acoustic focal plane, the deviation of 1mm relative to the focal field width of 2.5 mm affects the accuracy of HIFU treatment; when there is biological tissue in the acoustic channel, the ultrasonic beam does not spread, no defocusing phenomenon appears, and the focal width maintains in the range of 2.38- 2.79 mm; when HIFU penetrates biological tissue, the nonlinearity of acoustic focal region is obviously weakened; The higher the harmonic order, the greater the attenuation is. In the experiment, the acoustic attenuation values of the fundamental wave, the second harmonic, the third harmonic and the fourth harmonic are 9.97, 22.33, 28.05 and 31.06 dB respectively, and the fifth harmonic disappears due to severe attenuation.