Mr. Chen’s post also said that Mr. Wu had hired the lawyer Li Guifang, who is best known for defending the Chinese politician Bo Xilai in a blockbuster corruption trial in 2013.

If Mr. Wu goes through with it, an appeal will be a high-profile test of a court system that convicts nearly everyone prosecutors accuse and that often takes a dim view of open confrontation to its rulings. In pushing back, he could goad the courts into an even harsher sentence.

Mr. Wu’s company, Anbang Insurance Group, was seized in February in a case that offered a glimpse at some of the big risks hidden within China’s vast but troubled financial system. His prosecution was widely interpreted as an effort by Beijing to make an example of a firm that had become emblematic of a spree of debt-fueled overseas purchases by Chinese companies.

In the face of a possible life sentence, Mr. Wu had pleaded guilty to defrauding investors out of more than $10 billion and asked the court for a lighter sentence. This month he was sentenced to 18 years in prison.

Despite his guilty plea, Wednesday’s post represents the second time that Mr. Wu has disputed the Chinese government’s charges against him. In a March court appearance, he said he did not know whether what he had done constituted a crime, according to the court. But just hours later, it said, he pleaded guilty.