Phased array has the characteristics of steering focus and forming multiple foci. In this study, the Westervelt nonlinear acoustic propagation equation and the time reversal theory (TR) are used to calculate the pressure distribution of different focal regions in water which is based on a 64 element phased array by the finite difference time domain (FDTD) method. Simulation results demonstrate that the focal lengths formed by self focusing are less than the spherical geometry at 0.5~0.8 MHz. When forming single focus, the maximum pressures of foci decrease as the distance of off axis increasing. The ratio of intensity between side lobe and main lobe firstly decreases and then increases. When forming two foci at 0.7 MHz, the intensity of side lobe is low as the distance between two foci gets greater than 7 mm, and two foci fuse as the distance is less than or equal to 2.5 mm.