There is a 20kg limit for economy class, which the airlines are fairly strict about. Meanwhile business and first fares are also subject to (higher) baggage limits, but these are generally aren't enforced as long as you don't go too crazy.

That having been said, here a few comments:

-Consider leaving enough baggage in Beijing to duck under the limit
-It's usually possible to get away with large carry-ons
-Definitely buy two one-way tickets as opposed to a round trip, even if you don't require the flexibility this affords