Abstract: A numerical model is developed to simulate the acoustic field in heterogeneous tissue from a medical linear transducer. The coupled full-wave equation for nonlinear ultrasound is solved using a staggered grid finite difference time domain method. The distribution of acoustic pressure and power in human abdominal wall with heterogeneities in sound speed, density, and nonlinear parameter are obtained. Compared with homogeneous medium, when sound speed in tissue is uniform and density unchanged, the acoustic energy decreases only 1.8 dB in the focM region; when density in tissue is uniform and sound speed unchanged, the energy decreases 3.8 dB in the focal region, which is almost the same as heterogeneous tissue. Thus, the primary factor of the aberration of focused beam is the heterogeneous distribution of the tissue sound speed.