***** NOT A TEACHER *****
Hello, Xuzheng:
(1) Yes, the use of commas can be confusing to students and to us native speakers, too.
(2) Some people like to use the word "thus" in your kind of sentence:
The house is both simple and elegant, thus fulfilling its designer's dream.
(a) Some books, I believe, tell us that your sentence equals something like:
The house is both simple and elegant, a result that fulfills its designer's dream.
(3) Commas are often used to indicate a pause. In your sentence, you need the comma to indicate that another idea is coming next.
(a) For example, consider:
The airplane crashed in the field, killing all aboard.
That is "The airplane crashed in the field, an event that killed all aboard" or "The airplane crashed in the field, and the crash killed all aboard."
HAVE A NICE DAY!