这部电影改编于长篇小说。翻译

This film is based on a novel.

novel本身就是长篇小说。电影开头经常可以看到“based on true story”字样，意思是“根据真实故事改编”。

The film was adapted from a novel.

this movie was adapted from a river novel

The film is an adaptation of a river novel.