The 31-year-old McEvoy, who followed up his gold medal at the Paris Olympics in 2024 with a world title last year, clocked 20.88 seconds to eclipse the 20.91 set by Brazilian Cielo in 2009 during the 'supersuit' era.
McEvoy said he had been doing "special stuff" in training ahead of the China event.
"I knew I had a chance to do a PB. My old PB was 21.06, so maybe 20.99, but doing 20.88 is unreal. It's crazy," McEvoy told China's state broadcaster CMG.
"The 50m, I look at it as a strength-based skill. It's different to the other events in swimming. A lot more strength and power is involved, and men peak in strength into their 30s, well into their 30s."
The now-retired Cielo congratulated McEvoy on social media.
"Lightning fast swim! Incredible!" he wrote.
Cielo set his record in an era when a number of records were set by swimmers wearing high-tech suits, which have since been banned.