Abstract: Focusing into solids by transducer arrays and acoustic field distribution on focal axis are studied in this paper. The relation between source element width and field strength at different order of focuses is discussed. Numerical calculation is used to get focal field distribution when the element width changed. Some practical problems such as the minimum distinguished frequency, transversal and longitudinal resolution are investigated when this kind of focusing is considered to find applications in NDT and Acoustic Image. Some explorative experiments have been done to demostrate the theory.