Upon your departure via airport, you should take out the things that you had declared with the copy of your Customs Luggage Declaration Form which you filled in on your arrival. If any item is missing, a certificate by the relevant department is required (for instance, a certificate from the police is required if something is stolen); otherwise, you have to pay import duty according to the Custom regulations. 

China has strict regulations on some articles for export, such as cultural relics, medicinal materials, gold, silver, or antiques for export. The cultural relics that are forbidden to be taken out of China include antiques earlier than the Qing Dynasty and ancient paintings and calligraphy (also some modern ones). Keep the receipt of the antiques you bought so that you can show them on departure. These antiques bought in shops which can be exported usually carry a red wax seal. For those you may buy on free markets, you have to show them at the customs to avoid trouble. 

Travelers who want to change Renminbi (RMB) back to foreign currency at the airport before leaving China will be required to show the exchange slip provided when they exchanged foreign currency into RMB on arrival or at banks. 

Baggage allowance for air flights is 20 kg and hand luggage is 5 kg. Since baggage is still handled manually in many airports, it is advisable to bring strong suitcases with locks.
Airport tax for international flight is 90 yuan per person, for domestic flight is 50 yuan per person, which should be paid together with the ticket when it is issued. So you do not  need to pay any extra tax at the airport before check-in.