Sun, 33, returned to competition last year after serving a four-year ban for smashing vials of blood during a 2018 doping test, the circumstances of which he still disputes.
He also served a ban in 2014 after testing positive for a banned substance.
China’s week-long national championships in Shenzhen are the country’s sole qualifying event for the World Aquatics Championships in Singapore in July and August.
Sun will be ineligible because the Chinese Swimming Association (CSA) forbids those who have served a doping ban of 12 months or longer from competing for China.
There are no such restrictions applied to taking part in China’s national championships, where swimmers represent their provinces.
The 11-time world champion will face off against Pan Zhanle, the 20-year-old who broke the 100 metre freestyle record at the Paris Olympics last year on his way to winning gold in that event and the 4x100m medley relay.
Sun will compete in the 100m, 200m, 400m and 800m freestyle events, with Pan also entered for the 100m, 200m and 400m.
Sun blazed a trail for Chinese swimming when he became the country’s first male Olympic swimming champion, winning the 400m and 1500m freestyle at the 2012 London Games.
As a child, Pan named Sun as his inspiration and the two have never raced each other in competition.
In March, Sun finished second in 400m freestyle at the Chinese National Spring Swimming Championships in 3min 47.94sec – inside the World Championship qualifying standard.
Pan, who is more successful at shorter freestyle distances, set a 400m personal best of 3:45.58 at last year’s national championships.
In the women’s events at the China championships will be Zhang Yufei, who was one of 23 Chinese swimmers who tested positive for the banned substance trimetazidine in 2021 but escaped a ban.
She won six medals at the Paris Olympics last year in butterfly, freestyle and relay events. (AFP)
