Abstract: : High intensity focused ultrasound (HIFU) has attracted much attention as a new non-invasive cancer therapy technique. It focuses the ultrasonic energy into the target tissues inside the body, and the temperature of the target tissues rapidly increases in a short time, leading to the tumor cells coagulation necrosis with minimal impact on the normal tissues outside the focusing area. The phased array technology has some advantages of electronic beam focusing and multi-focuses, it is required to control the exciting vector and the phase of the elements based on the depth and size of the lesion area. Phased array is inevitably influenced by grating lobes in focused ultrasound therapy. Based on the superposition technique of Gaussian beams combined with the inverse matrix algorithm, the sound field generated from a phased array is calculated, and the optimization study of grating lobes is developed. The parameters of the phased array elements are first estimated from the focal parameters using the inverse matrix algorithm; Then the elements are expressed as a set of Gaussian functions; Finally, the sound field can be calculated using the superposition technique of Gaussian beams. A 9×9 two dimensional phased array is performed in the numerical simulation. In the linear case, the difference between the results of the Gaussian superposition technique and the Fresnel integral is less than 1%, which verifies the feasibility of the approach. The side-lobes generated from a 2-D phased array can be effectively suppressed through the decrease of element spacing in a single focus mode, but cannot be fully suppressed. The theoretical approach will be improved by further experimental works. This study will promote the application of the phased array in HIFU.