Abstract: An acoustical image can be numerically reconstructed by several methods, one of which is the Backward Propagation method （B-P method）. When the imaging distance is very small, the lateral resolution will be constrained by the spatial procedure sampling spacing if the traditional procedure of B-P method is used. This problem is discussed and a solution, called the method of increasing value in the spatial frequency region, worked out. Some discussion about high-order images is also given.