Jelle van Gorkom suffered a damaged skull while training at the national sport centre at Papendal in the eastern Netherlands.
The 27-year-old BMX star also sustained broken ribs, a fracture to the face and damage to his liver, spleen and kidneys.
Officials say the accident happened at the beginning of his training, at the bottom of the starting hill.
Jochem Schellens, the sport centre's director, said: "It is a terrible accident and at the moment we are mainly with our thoughts at Jelle van Gorkom, his girlfriend and his family and we wish them all the strength in the coming period."
This is not the first time that Mr van Gorkom has been injured while competing.
He was unable to compete for medals during the London Olympics in 2012 after suffering a "huge injury" in the run-up to the Games, according to his website.
He also broke two collarbones at the end of 2015, and suffered another break in March 2016.