It was the 21-year-old’s third successive Superseries win in as many weeks after her triumphs in India and Malaysia.
Ratchanok, the 2013 world champion, will displace Spain’s Carolina Marin as number one when the weekly world rankings are updated Thursday.
Despite losing 18-21 in the first set as the lanky Sun Yu put her superior reach to better use, Ratchanok coasted to a 21-11 victory in the second.
She then claimed the third set 21-14 as her opponent appeared rattled by several errors.
“I played well at the beginning but I got nervous when I was leading,” Sun Yu, ranked 14th in the world, told reporters after the match.
Ratchanok, who has now established herself as one of the favourites for Olympic gold, said the win would give her a boost for Rio.
“Now I am more confident to get a gold at the Olympic games,” she said. “I thought that today would not be my day and she was playing well. My body was tired from the three weeks but I just wanted to do my best.”
