I don't know why has this problem but I can find many � in my project. I can't find the rule and It always confuse me!
Sounds like your chosen font can't represent the characters you're typing.
Thanks for your answer! I have a question: If the font can’t represent the characters, all same characters will become �. Is it right?
No, not necessarily.
You might have to read up on unicode. Characters are based on ranges and converted accordingly to such ranges.
However, I tend to agree 100% with Printninja.