What's the shortest way from Bucharest, Romania to Haikou (through Beijing or Shanghai or else?)

From Bucharest, you could fly into Hong Kong, and then transfer to Guangzhou by bus. Finally, take a train or flight to Haikou.

The shortest and fastest way is to fly from Hong Kong to Haikou directly. If there is no direct flight from Hong Kong, my advice for you is to fly from Bucharest to Singapore, then to Haikou by direct flight with Jet Star or Tiger Air. Many airlines fly from Europe to Singapore.