are chinese nationalities can only have a relationship and marry the same nationality?

Answer 1

If you mean "Are Chinese permitted to marry non-Chinese?", the answer is yes. If you mean "Are Chinese inclined to marry non-Chinese?", the answer is up to each person. Most Chinese would not want to marry a foreigner as their culture is nearly fixed. They and their families don't change their culture and don't consider a foreigner to be any part of their culture so the foreign spouse would never fit in smoothly.

If you say "nationalities" when you really mean "56 ethnic groups", as is commonly said in China, the answer is similar to the above about foreigners but less severe. In particular, the muslim and Tibetan groups are considered more "foreign" than are the others who are mostly Han or Han-mixed. As a matter of fact, the people of Hui "nationality" are Han-mixed even though muslim so they are more accepted into the Han culture than are Uyghurs, who are not at all related to any Chinese groups.