@zhuxiaohong
Why do British call fall "autumn"?
A far better question would be “why do Americans and Canadians call Autumn ‘Fall’?” They are about the only countries that do - the rest of the English-speaking world says Autumn.
Autumn comes from the old Latin name for the season between Summer and Winter, and has been the most popular name for the season in English - in one form or another - for at least a thousand years. The term Harvest was also a common term for the season during Medieval times, but gradually came to be used specifically for the activity of gathering crops rather than the season as a whole. In the north of England the dialect term Back-end was also once quite common.