If you want a cheap flight from Bangkok to Beijing, you've got two airlines to choose from.
Beijing Capital Int. Airport is just one of the airports in Beijing. If you're not set on a specific airport, try changing your search from Beijing Capital Int. Airport to Beijing to see a broader range of flights – they might be cheaper. It usually takes about 13 hrs 5 mins to fly 3319 km to Beijing Capital Int. Airport.
There are about 44 flights to or via Beijing every week, so you have plenty of options.
If you're planning on going to Beijing during this time, you could take a look at other airport options. Or you could try to get a last-minute bargain.