Petition for modification of Child Support BEFORE she moves away

You can also petition to modify custody agreement, since she will have less access to the child.  Basically, this is a chance to review the child issue with the court in a new light.  The parent who stays with the child would gain a lot in the process. 

You have to settle these issue before the mother moves away.  When she's in China, the court cannot do anything.

Here is why modify custody agreement is important.  Once she's in China, she could refuse to pay, you can't do nothing.  But I assume at some point she'll come back to see her Child, you can refuse her visitation if she does not pay.