You could use "no problem" here, Weiping. It would be a shorter way to say "What you did is not a problem. I'm not hurt." "Forget it" is also fine. If you really wanted to be nice, you could even say "That's all right. You didn't hurt me at all."
Forget it would seem a little too dismissive, the other person is already being deferential by saying sorry, so using an imperative phrase might be seen as rude. Depending on the tone of voice, etc. it might even be seen as sarcastic, i.e. you haven't really forgiven the person.
No problem is usually when someone thanks you for doing something, but really the two are both just set phrases that act as social lubricants, rather than completely specific responses. No problem would be preferred.
Much more information is conveyed in body language and tone of voice here than the actual words.