Taiga Hasegawa of Japan took silver, while Jake Canter claimed bronze for the United States.
Su, the slopestyle silver medallist at Beijing 2022, started with an impressive first run for a score of 82.41 under sunny skies in the Alpine town of Livigno and could not be caught.
The Chinese rider ripped off his hat and clapped his hands when he realised he had won gold. Su shed tears as his coaches and competitors congratulated him. "Let's go," yelled Canter.
The slopestyle contest had been delayed by a day because of a snowstorm on Tuesday.
Su won bronze in the men's big air last week, having claimed the title four years ago. The 22-year-old said he felt "crazy pressure" in that event as the defending gold medallist.
In slopestyle, riders navigate rails and other obstacles and perform aerial tricks to impress the judges with difficulty and originality. The best score from three runs determines the winner.
Mark McMorris, who has three Olympic slopestyle bronze medals, finished eighth after crashing on two of his three runs.
The oldest competitor in the field at 32, the Canadian suffered a concussion at the start of his fourth Games and had to skip the big air event.