The synthetic aperture (SA) imaging based on multi-images fusion is dynamically focused in both transinission and receiving yielding an improvement in resolution. But this imaging technique sets high demands on processing capabilities, data transport and storage. It also makes the implementation of a SA system very challenging and costly. The proposed method is specifically for this issue. The method uses the focused image lines as input data, and takes advantage of the synthetic aperture imaging based on multi-images fusion which picks out the coherent samples of the recorded echoes and sums these samples, thus obtaining a high resolution image. The proposed method is dynamically focused in both transmission and receiving, and a range independent lateral resolution is obtained. With Field II software, simulations on scatters and phantom are made and compared with broadband minimum variance (MV) beamforming and SA imaging based on multi-images fusion, the feasibility of the method named a fast synthetic aperture (FSA) imaging is verified.