Congratulation!
You need tell them which day you will start (they let you make the decision). The background check and other checks will going on after you accept the offer. but, as you know you are good, so that will not be a problem.
Don't forget to do the final negotiation about your benefit before you accept the offer, everything can be talked at this time, including salary, stock option, vacation days, on job training... depend on your negotiation skills ...
Usually not many Chinese know how to negotiate at this moment. What they usualy do is just sign the letter and go to work.