Chung Biu "Bill" Yuen, 66, and Chi Leung "Peter" Wai, 41, who worked for the UK Border Force, were convicted in May of assisting a foreign intelligence service by carrying out surveillance on targets between December 2023 and May 2024.
They are believed to be the first people to have been convicted of spying for China in Britain.
Wai was also convicted of misusing his Border Force job to search the interior ministry's computer database for details about targets.
The men, both dual Chinese and British nationals, had denied the accusations, while the Chinese embassy in London said the case was "nothing but a political move of abusing the law".
"The United Kingdom now faces persistent, adaptive, and often clandestine interference by foreign state actors and those acting on their behalf," the judge, Bobbie Cheema-Grubb, told London's Old Bailey court.
"Modern foreign intelligence activity is not confined to orthodox espionage against military or governmental secrets. It may take the form of surveillance, information gathering, intimidation, and targeting of dissidents and those who have sought the protection of this country's laws."
She jailed Yuen on Thursday for eight years while Wai was sentenced to 10 years in prison.
"The activity of Wai and Yuen was truly chilling," said Commander Helen Flanagan, head of counter terrorism policing in London.
"They were spying and targeting individuals in the UK who were pro-democracy campaigners and were simply protesting against the Hong Kong and Chinese government and authorities."
