Abstract: Spherical cavity transducer is a special focused ultrasound transducer. In order to theoretically prove that it could break through the limit of the dimension of focal region and focal gain in conventional focusing ultrasound, a numerical model is developed for describing the nonlinear acoustic field generated from the spherical cavity transducer using the finite difference time domain (FDTD) method to solve the nonlinear Westervelt equation. Numerical simulation is performed using a spherical cavity transducer with diameter 120ram working at 0.6 MHz, and results are compared with those obtained from the conventional curved transducer with the same configuration. Results indicate that as the exciting acoustic pressure is 100 kPa, the spherical cavity transducer could increase focusing gain by about 8.5 times and reduce -6 dB focal region by 20 times in z direction. This study suggests that the spherical cavity transducer will be promising in the application of High intensity focused ultrasound (HIFU) for precise treatment.