Qin and He Chong led throughout at Barcelona's spectacular hilltop Montjuic pool, with Russian pair Evgeny Kuznetsov and Ilia Zakharov taking silver and Jahir Ocampo and Rommel Pacheco of Mexico bronze.
He Zi, by contrast, was trailing Italy's Tania Cagnotto going into the last of her five dives but conjured the best effort of the day, a reverse 1-1/2 somersaults and 1-1/2 twists, to win by one tenth of a point, the smallest victory margin at a world championships.
He Zi finished on 307.10, with Cagnotto on 307.00 and He's compatriot Wang Han winning bronze with 297.75.
"Before the last dive I was really nervous because the scores were very close," He Zi told reporters.
"I managed to stay focused and I got the gold," she said. When told her victory margin was the narrowest ever, she added: "I couldn't imagine. Knowing that gives more value to my gold medal."
It was a fresh cruelty for European champion Cagnotto, who was denied a medal at the London Olympics last year when she finished two tenths of a point behind third-placed Laura Sanchez of Mexico in the three-metre springboard.
Her father Giorgio Cagnotto won four Olympic diving medals in the 1970s and she has won multiple world championship silver and bronze medals but has never stood on the podium in the four Olympics for which she has qualified.
She did not seem too disheartened by Tuesday's near miss.
"Diving results shouldn't be so close because sometimes it hurts," she told reporters.
"At the Games, I was really disappointed, here I am happy," she added.
"After the London Olympics I took a break and I came here not so well prepared as in previous major competitions.
"It was the first time I got so close to the Chinese. This is a good sign, I am getting closer to them."
Qin, a double Olympic champion in three-metre synchro, secured an unprecedented fourth straight world championship triumph on Tuesday after he also won the event in 2007, 2009 and 2011.
He Chong, Olympic champion in Beijing in three-metre solo, won the world title in the synchro in 2005 with Wang Feng.
China swept all 10 diving golds at the last world championships in Shanghai two years ago but Germany's surprise success in the men's 10-metre synchronised platform on Sunday means they can only win nine this time around.
(Writing by Iain Rogers in Madrid, editing by Toby Davis)
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