Sandy Phan-Gillis, who has Chinese ancestry and is a naturalised US citizen, was arrested in March 2015 while about to leave mainland China for the Chinese-ruled, former Portuguese colony of Macau, and had been held without charges since then.
She had plead guilty during a trial in the southwestern city of Nanning and was not planning to appeal, lawyer Shang Baojun told news agency Reuters.
"She will probably be exported to the US soon, but we do not know the exact date yet," Mr Shang said, adding that she was being held in a police station in the meantime.
The government has not released details of the charges against Ms Phan-Gillis.
Mr Shang said that because the case touched upon 'state secrets', he could not reveal details of the ruling until the official verdict had been released, which was expected within the next five days.
Ms Phan-Gillis had been in regular contact with US officials while in detention, Mr Shang said.
The US Embassy in Beijing referred questions about the charges of espionage to Ms Phan-Gillis' legal team.
"We remain concerned about Ms Phan-Gillis' welfare and continue to follow her case closely," embassy spokeswoman Mary Beth Polley told Reuters.
Calls to the court went unanswered after work hours on Tuesday, and it was not possible to reach Ms Phan-Gillis for comment.
China's definition of state secrets is very broad, encompassing everything from government directives to top leaders' birthdays. Information can also be declared a state secret retroactively.
The arrest added to tension between China and the United States in the final months of former President Barack Obama's administration, a relationship already strained by China's assertiveness over its claims in the disputed South China Sea.
Despite criticising China on the campaign trail, US President Donald Mr Trump has recently warmed to it after a summit with President Xi Jinping in Florida in early April when Mr Trump said they had developed an 'outstanding' relationship.
In recent days, Mr Trump has praised Chinese efforts to press North Korea to give up its development of nuclear weapons and long-range missiles.
(Reporting by Christian Shepherd and Michael Martina, Editing by Robert Birsel and Michael Perry)