Well, CA domestic is as reliable as USA or European domestic flights--generally OK or a little late. But no guarantees. Depending on when you are traveling, could be weather, or mechanical delay, etc. Chinese domestic airlines are known to cancel flights due to low loads, and reschedule passengers onto another flight. This could present a risk to you, as I thought Air China only has one flight a day DNH-PEK, and if it is cancelled for any reason, you could have a problem. In your shoes, I would try to leave Dunhuang a day earlier to allow a cushion. But if the flight and cruise schedule days are fixed and immoveable, I would have one or more backup plans in case things head into the weeds:

1) Get a full fare Air China ticket that can be endorsed to another airline. Sometimes the deep discounted ones are only good for the issuing airline. There aren't many others serving DNH, and they may not fly to Beijing, but certainly to Xi'an, Lanzhou, and I think even a Tianjin flight. The point being: GET EAST so you have more flight options, even if they require a connection and getting to Beijing/Tianjin late at night. Come to think of it, you might want to go ahead and purchase backup (refundable tickets) on alternate airline and flights to make sure you are covered.
2) Do some advance research on train schedules, esp overnight from Lanzhou or Xi'an, in case your "backup flight" can only get you part of the way. If weather delay grounds all flights all day in DNH, you are screwed, as train from Dunhuang-Beijing/Tianjin is just too far to make in time you have.
3) The morning of your departure from DNH, try to get some info on whether flight is going or not (this is easier said than done in China). If flight is reasonably full and weather is OK, you should be OK. Otherwise, put Plan B into action without hesitation, before others on your flight do same. Chinese airlines are notorious for non-communication and stringing people along with promises of short delays, only to have them turn into hours-long delays and finally cancellations.
4) If all else fails and you can't get to Tianjin to make your cruise, find out where it is going next and figure out how to meet them at next port of call (I'm assuming this is in China or other NE Asia!). Do the prep work in advance. This is likely to be an expensive "salvage operation".