I will be traveling to China early next year. My question is this: How can I fulfill my Sunday obligation to attend Mass while in China? The Catholic churches there are either under the control of the government or are in hiding.
You cannot seek out the underground churches in China because to do so would put innocent people at risk. When Catholics are traveling in areas where they cannot attend a Catholic church that is in union with Rome, they are dispensed from the Sunday obligation to attend Mass. You can though, if you wish, attend Mass at a state-approved Patriotic Catholic church and receive Communion there. Canon law states:
Whenever necessity requires it or true spiritual advantage suggests it, and provided that danger of error or of indifferentism is avoided, the [Catholic] Christian faithful for whom it is physically or morally impossible to approach a Catholic minister are permitted to receive the sacraments of penance, Eucharist, and anointing of the sick from non-Catholic ministers in whose churches these sacraments are valid
(canon 844 §2).
The sacraments are valid in the state-approved Catholic churches in China, so you may receive confession, Communion, and the anointing of the sick from priests of the Patriotic church during your stay in China.
Masses in China by Jimmy Akin