The entropy is an important factor to estimate whether the digital image is basically the same with the original image. Usually, the higher the resolution is , the more similar the digital image is to the original one. We establish the fuzzy sets and the membership function which can be consistent with the definition by the gray degree of the black and white image. We calculate the entropy by the Shannon Entropy. For a series of resolution, we can get the entropy of the fuzzy sets which can reflect the definition of the image. The relation between the entropy and the resolution can be constructed by the logistic predicting model. The entropy of an original image can be predicted by the relation. The example shows this method is effective and the result supplies the theory base for the confirmation of the resolution.