Lu was the gold medal winner four years ago and Dong won the bronze. Things were reversed this time, Dong winning with a score of 62.990.
Russian Dmitry Ushakov finished second and Lu was third.
The Chinese gymnasts also finished 1-2 at the most recent world championships, which Lu won.
Canada's Jason Burnett, who finished between Lu and Dong in second place at the Beijing Olympics, was eighth in Friday's final.
This was the fourth time an Olympic gold medal had been awarded in trampoline. Prior to the Chinese becoming a force in the sport, the golds were won by Russia and Ukraine.