In the women’s singles final, Japan’s second-seeded Akane Yamaguchi beat Indonesia’s fifth seed Gregoria Mariska Tunjung.
Li, ranked ninth in the world, won 21-10, 21-13 to see off world number 31 Leong, who was appearing in his first Super 500 final on badminton’s world tour.
Li beat Denmark’s double Olympic champion Viktor Axelsen in the semi-finals and he was again in superb form in the final, winning in 44 minutes.
The 24-year-old celebrated with an acrobatic flip on the court.
“Mostly things went to plan,” Li told the Badminton World Federation.
“Leong is powerful, both of us wanted to win the title. It was my goal to win and it’s a really important experience for me.”
Leong beat a host of top players this week in Japan, including an epic semi-final comeback win over Indonesia’s fourth-seeded Jonatan Christie.
But all four of his matches on the way to the final went to three games, and he looked like he had little left in the tank to face Li.
“I did my best,” said Leong.
“In that match I gave it all I had to be my best today.”
Yamaguchi cruised past Tunjung 21-12, 21-12 in the women’s final.
It was the two-time world champion’s second title of the year, after winning the Japan Open in Yokohama in August.
“Playing at home motivated me a lot because of the fans, and my parents were here,” she said.
“I heard them cheering for me and that gave me extra energy.”
In mixed doubles, Thailand’s Dechapol Puavaranukroh and Supissara Paewsampran beat France’s Thom Gicquel and Delphine Delrue 21-16, 10-21, 21-17.
China’s number two seeds Liu Shengshu and Tan Ning took the women’s doubles title, beating Japan’s Yuki Fukushima and Mayu Matsumoto 21-15, 21-5 in the final.
Indonesia’s number three seeds Fajar Alfian and Muhammad Rian Ardianto triumphed in men’s doubles, beating Japan’s Takuro Hoki and Yugo Kobayashi 21-15, 17-21, 21-17.
By AFP