Taiwan has charged two former state officials with violating the national security law by organising meetings between former senior military officers and Chinese intelligence personnel.
Those encounters allegedly allowed the Chinese to "contact and even recruit" ex-military officers to their network, BBC reports.
Prosecutors say the pair arranged 13 free trips to mainland China for 48 former officers from 2013 to 2018. The men earlier denied they were recruiting spies for China.
The meetings also promoted China's unification with Taiwan, prosecutors say.
Retired rear admiral Hsia Fu-hsiang and ex-MP Lo Chih-ming face up to five years in jail if convicted.
China considers self-ruled Taiwan as a breakaway province and has vowed to place it under its control, by force if necessary.
Taiwan's strongest ally, the US, warned in October that China is pursuing unification at a much faster timeline. At around the same time, China's leader Xi Jinping embarked on a historic third term as head of both the Communist Party and the military. Beijing and Taipei have spied on each other since the end of a civil war in 1949 that saw the Communist Party take control of the mainland while nationalists settled on Taiwan island.
Prosecutors say Mr Hsia and Mr Lo have been involved with Chinese organisations that advocate for unification since 2013. The two men have been detained since January.
But there is no evidence showing the retired officers who joined these trips collected confidential information for China, so they are listed as witnesses, prosecutors say.
Their names surfaced as prosecutors investigated an air force colonel who allegedly recruited at least six current members of the Taiwanese navy and air force on behalf of China for over eight years.
"We urge retired military officers to exercise caution regarding unreasonable benefits when they go to events in China to avoid falling into the traps" of Beijing, prosecutors say.
A number of former high-ranking Taiwan military officials have been accused of aiding Chinese intelligence in recent years.
In January, a retired air force major general was found guilty of accepting meals and trips from a Hong Kong businessman who was acting on behalf of Beijing. However, his sentence was suspended since he showed remorse and had no previous criminal record.