What happen when the polarity of transformer reversed?

You cannot 'reverse' the polarity of a transformer -it is either wound with 'additive polarity' or 'subtractive polarity', and there's not much you can do about it!

For a single transformer, its polarity doesn't really matter. But if you are going to parallel two Transformers, then you must know the polarity of each transformer in order to avoid harmful circulating currents in their secondary windings.