Assuming your incoming and outgoing flights are on time, you should be OK. Beijing Airport has 3 terminals, and fortunately your connection should be taking place in the same one--Terminal 2. Hainan Airlines INTERNATIONAL flights go in/out of T2 (domestic HU are out of T1)...and ALL China Southern CZ flights are out of T2.

Allset2travels comment on "vast" applies to T3 only. Fortunately, your terminal T2 is an older and smaller terminal which is less time-consuming to traverse, and it doesn't have the volume of international flights coming in and out anymore, as most were shifted to new T3. I just flew intl into T2 today from HKG, and there were NO lines at immigration or customs, and baggage delivery at PEK T2 is usually pretty speedy. I made it from door of aircraft to exit door of terminal in 20 minutes, and that included pickup at baggage claim belt and Customs. I would think if you allow 30 minutes for the entry formalities and bag pickup, you'll be covered. One caveat: though no longer inundated by intl flights, T2 also doesn't have mountains of immigration staff around, so if your flight arrives at same time as another intl flight full of foreigners, it's possible the few open queues for non-Chinese could get backed up. If this happens to you, beg for mercy, plead your case, and see if others will let you queue-jump. Most foreigners arriving in Beijing are unlikely to have connections, or at least not immediate connections that are as time-sensitive as yours.

Don't expect to be able to interline bags in BRU all the way to Xining. Even if these two unrelated airlines would cooperate and do it (they don't and won't), they'd likely find a way to screw it up. So, do it one step at a time, pick up bags in Beijing, go through Customs--usually is nothing more than walking past the agents, and there are no longer forms to fill out and turn in. Then out into arrival hall, and up the escalator or lift to departure level, head for domestic departure check-in and look for the bank of monitors showing your airline/destination/check-in desk. It should take you no more than 10 minutes to do the bag pick-up-Customs-up to check in desk shuffle. Pull the BRU tags off your luggage so you don't confuse the baggage handlers. You need to be in that check-in line 45 minutes ahead of flight departure. So, add this up and you have about 15 minutes spare time in there. Not a lot, but doable. Once you get checked in for domestic flight, it only takes about 10-15 minutes to get from desk through security and to boarding gate. At BRU check-in, try to get an aisle seat on the aircraft as close to front as possible, and pray when you arrive at Beijing, you get a jetway and not a bus gate. Hoof it as quickly as possible to immigration...especially if there are a lot of foreigners on the flight who will be needing the same queues as you.