Regional energy focusing is a crucial technology for precision energy delivery. This paper mainly focuses on one of its typical applications: PRecision Electronic Warfare (PREW), and a new regional energy focusing method under ultra-sparse array is presented. By modeling regional energy focusing under ultra-sparse array as a Semi-Definite Program (SDP), an optimization can be made for self-correlation matrix of the transmitted waveforms to provide a focal spot matched with the entire target region, meanwhile, to minimize the energy level on the surrounding interesting region. Finally, four assessment indicators are proposed to evaluate energy focusing effect. The numerical results indicate that the proposed method, with more practicability, can provide a better energy focusing performance than the existed methods under typical arrays.