The 30-year-old, who also won gold in the event at the London 2012 Games, said concentration was the key to his victory.
"I got gold again after London, which is good. It is about your own condition and own performance and your own focus.
"My family will be happy, but for them it is not about me winning a gold. If I did not win gold they would still be very happy with my participation."
Dong's left leg was amputated after was hit by a car when aged three.