Hello @ljjiayou,
The value computed in ITK/SimpleITK is mutual information (not normalized).
The bounds on MI were provided in the response to one of your previous questions.
Even if two images are exactly the same the MI will most often not be 1, please look at the MI formula, equation 1 on that page, to gain a better understanding of this similarity metric.
Comparing MI before and after registration will only show you that the registration is minimizing the negative mutual information (value after registration will be lower than before, ITK/SimpleITK works with -MI). Please do not use MI to quantify the quality of the registration, it is not reliable (for details see the paper by T. Rohlfing).