Beijing claims self-ruled Taiwan as its territory and has ramped up military and political pressure on the democratic island in recent years.

The two sides split in 1949 after a civil war and have been spying on each other since.

Lou Wen-ching, a former instructor at the Air Force Flight Training Command with the rank of lieutenant colonel, was convicted of violating the Criminal Code of the Armed Forces for "rendering aid to an enemy's spy" and delivering "military secrets" to an enemy, the court said.

"He was introduced by a Chinese Communist Party (CCP) spy surnamed Liu (a retired Air Force lieutenant colonel) to meet with CCP officials abroad," it said in a statement.

He "assisted Liu in engaging in espionage activities by providing internal documents of the Air Force (some of which were categorised as confidential) for rewards".

The court did not elaborate on what kind of material Lou had passed to China, saying it could only offer a brief explanation as the case involved "defence secrets".

It added that Lou's criminal proceeds totalling $5,580 were confiscated.

Lou was previously acquitted twice in past rulings by the High Court.

Monday's statement did not specify why the court reversed its decision, but said Lou was considered to have "served the purpose of benefiting the enemy", whether his actions were out of "direct or indirect intention".

The sentencing on Monday was the latest in a recent string of spying cases.

Last month, the High Court sentenced eight defendants to up to 13 years in prison for spying for Beijing for money when they were active-duty soldiers.