Luo Changping, who now runs a legal consultancy, was a well-known investigative reporter.
Police in the Jiyang district of Sanya identified the 40-year-old man as Luo Changping in a statement on Friday. It said Luo was being held on the charge of "infringing the reputation and honour of national martyrs" and the case was being investigated.
China introduced a law in 2018 making it a criminal offence to defame or deny the deeds and spirit of the country's historic martyrs.
Luo was detained two days after he posted a comment on social media about a new Chinese film, The Battle at Lake Changjin, saying troops who died in freezing temperatures in a 1950 battle portrayed in the movie were "stupid".
Those soldiers are hailed as national martyrs in China for their sacrifice in the Battle of Chosin Reservoir, and Luo's post on Weibo drew criticism from thousands of social media users.
The People's Liberation Army official newspaper, PLA Daily, also responded with a commentary the same day calling the soldiers an "indelible symbol" of China's role in the Korean war.
"Heroes should never be ridiculed, stigmatised and insulted," the commentary said. "We must not allow those who maliciously slander heroes to act unscrupulously and do whatever they want."
The blockbuster about the Korean war was the best performing movie in China during the holiday week.
It stars Wu Jing, who is best known for his roles in the nationalistic Wolf Warrior franchise.
Luo removed his post from the social media site on Thursday and apologised for the "inappropriate comments".
In the statement, Jiyang police said they had received "many complaints from the public" about Luo's post, which it said had "a very bad influence".
A well-known investigative reporter, Luo exposed the corruption of former National Development and Reform Commission deputy head Liu Tienan in 2012, who was later jailed for life for taking bribes. Luo has since left journalism and started a legal consultancy business.
His case follows that of Qiu Ziming, a popular blogger who was jailed for "defaming martyrs" in June after he suggested the death toll of the China-India border clash last year was higher than the official count of four and questioned the PLA's resolution in defending China's border. Qiu, who had more than 2.5 million followers on Weibo, was sentenced to eight months in prison.
William Zheng is a veteran journalist who has served and led major Hong Kong and Singaporean media organisations in his 20-year career, covering greater China. He is now a senior correspondent on the China desk at the Post.
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