During the defense, how to prove the method I used in my master thesis is correct?
This is not something you should get to your defense with your advisor still doubting. You and he need to sit down ASAP, work through the material, and not stop until he is satisfied. I would would hope that he would be willing to take this very seriously. No advisor should let a thesis go to a public defense with this kind of question looming in their mind. No one should be forced to do a defense that their advisor thinks they're not ready for.