1. An ultrasound system for forming an ultrasound image by using a plurality of sub-frames, comprising: a plurality of transducers for transmitting and receiving ultrasound signals along a plurality of scan lines to obtain sub-frames; a determination unit for locating a common point where extensions of the scan lines with respect to the transducers intersect and determine a plurality of virtual common points based on the located common point, the determination unit further being configured to determine a plurality of transducer groups, steering angles and a plurality of scan-line groups; a controller for being responsive to a change of the sub-frames to select scan lines corresponding to the changed sub-frame and adjust steering angles of the selected scan lines based on the plurality of transducer groups, the steering angles and the plurality of scan-line groups; and a processor for synthesizing the plurality of sub-frames to form an ultrasound image.