According to the properties of the square, we can get AB=AD=CD, ∠BAD=∠CDA, ∞△ADG =∞△CDG, and then we can prove that △ABE and △DCF are congruent from "edges and corners", and we can get ∠ 1 = ∠ according to the congruent triangles correspondence angle. Then ∠ AHB = 90, take the midpoint O of AB and connect OH and OD. According to the fact that the median line on the hypotenuse of a right triangle is equal to half of the hypotenuse, OH= semi-AB= 1 can be obtained, and OD can be obtained by pythagorean theorem. Then according to the trilateral relationship of the triangle, we can know that when the straight line of O, D and H is three points * * *, the length of DH is the smallest.