Client has asked me to trade mark Brand X for use on a web site. There are no competing marks on the CNPTO but somebody already has a website up called Brand Xing (the same name as my client with an "ing" on the end). As an experienced China intellectual property lawyer, we often face such tough issues that unspecified by law.

They provide exactly the same service.

My client's mark is not in the stream of commerce. The competitor's is in the stream of commerce. In other words, the competitor is already using it.
We haven't used it yet.

If I get our trademark registered, does that mean we can force the competitor to stop using it even though he was using it first?

We've got a couple of issues here.  First, whoever uses a mark first has priority, so my client's trademark filing won't stop the prior user of the mark, it will only (possibly) serve to stop that entity from expanding its use of the mark beyond where it's doing business now.

Second, even if we obtain a registration for my client, that registration will be subject to attack a user of the mark with priority.

Finally, it's not clear just how close the marks are from my description, but a trademark applicant is supposed to verify, under penalty of perjury, that its right to use the applied-for mark is exclusive.  Making such a claim when we know otherwise is arguably fraud on the PTO.

If we think we can make a good-faith argument that the marks are not confusingly similar, we can proceed, but at a minimum my client is going to be looking over its shoulder at this prior user for some time.