A nonlinear dynamic focusing control method includes: (1) using a minimum length in a scan line having the minimum length and a maximum length as an initial location and assigning a focal point on the scan line; (2) calculating half depth of focus of the focal point, and the initial location plus the half depth of focus of the focal point being the location of this focal point on the scan line; (3) determining an initial calculation location of the next focal point by adding the half depth of focus of the focal point from the location of this focal point on the scan line in the direction of the maximum length; and (4) determining if the initial calculation location of the next focal point is greater than the maximum length.