The 16-year-old Grand Prix debutant topped the podium at the summer event with jumps of 92.5 and 96.5 metres, which gave her an overall total of 221.9 points.
Austria’s Daniela Iraschko was second 1.2 points behind, with team-mate Jacqueline Seifriedsberger third.
"I showed a really good jump in training and my timing at take-off is very good," Pretorius said afterwards.
"It was so much fun today and hopefully we will have more competitions here in the future."
It is the first time such a meeting has been held at the resort.
The women’s Summer Grand Prix will continue at Hinterzarten in Germany on Friday and Saturday.