They are to an extenct interchangeable. Both refer to physical appearance. Beautiful is probably perceived as being more attractive than pretty. But if refering to a young girl you would probably use pretty rather than beautiful. You can say pretty flowers and also beautiful flowers.

beautiful 1， 形容人外表美丽漂亮 2，形容事物美好悦目/令人愉快的 3，某事完成得非常出色

1, a pretty woman is good-looking in a fairly ordinary way & has a nice face. 2, a beautiful woman, beautiful is a stronger word, used to describe a woman (girl) who is unusually attractive & has perfect good looks.