The photographer, who was thrown to the ground and kicked, reportedly suffered fractured bones in his face and ruptured vessels in an eye.
It has caused anger in South Korea, where the main opposition has called for Mr Moon to cut short his visit.
Koreans also say Chinese leaders have repeatedly snubbed Mr Moon.
President Moon’s visit was intended to improve relations, which had cooled after South Korea agreed to have a US missile defence system located on its territory to defend against possible attack from North Korea. China says it is a threat to its security.
The attack on the South Korean photographer took place on Thursday as Chinese security prevented South Korean photographers from following Mr Moon’s delegation at a trade show in Beijing.
He and another photographer, who was less seriously hurt, were due to return to South Korea on Friday for further treatment.
Chinese foreign ministry spokesman Lu Kang said there was “concern” at the injury but added that the event had been organised by South Korea, which had also hired the security guards.
The South Koreans should have “found an equilibrium” between the media’s needs and the guards’ “professional requirements… [to] sufficiently guarantee the safety and dignity of the people they protect”, Mr Lu said.
South Korean politicians, media and social media users have all expressed fury at the incident, as well as what they see as a series of diplomatic slights.
Mr Moon had his first three meals in China without any Chinese officials present, a lunch with Chinese Premier Li Keqiang was cancelled and he was met off the plane by an assistant minister, unlike other regional leaders, South Korean media said.
Kim Dong-cheol, leader of the second-largest opposition People’s Party, said the visit had so far been a “series of mistreatments, humiliations and shames”.
The country’s biggest newspaper, Chosun Ilbo, said the “lynching” and Mr Moon’s treatment were intentional and caused by Chinese “arrogance” and Seoul’s “subservient attitude”.
However Mr Moon, Chinese President Xi Jinping and Premier Li have all said they are ready to reset ties.
“Both sides are looking forward to the warmth of the springtime,” said Mr Li. (Courtesy BBC)