Nathan Law was speaking from London, where he is claiming political asylum after leaving Hong Kong last June.
Even though he said he no longer feared arrest, the 27-year-old said he still did not feel completely secure.
"Of course, you always feel unsafe," Mr Law said, speaking on Sky News's Into The Grey Zone podcast, which this week explores a battle of values between China's ruling Communist Party and democracies like the UK, US and Australia.
"We're talking about the largest authoritarian regime in the world and the most sophisticated surveillance autocracies that we have. And they have so many [ways] to persecute or hunt down people that they don't like," he said.
But Mr Law said he felt he still had to keep up his work campaigning for democracy in the former British colony, especially because a number of his fellow-activists are in jail or facing potential prison sentences.
Mr Law said it was the expectation he would fall foul of the new legislation that prompted him to leave his home city.
He said he has cut ties with his friends and family out of concern that they could face problems just for being in touch with him.
"And it's not fun at all, being out of the place that you love," Mr Law said.
"So I guess there are many, many things that we carry even though we leave Hong Kong. And I really do hope that every bit of action that I'm taking now can help and can pave the way for me for my way back to Hong Kong."
The UK has condemned China over its implementation of the law in 2020, claiming it violates an agreement between the two countries that was signed before Hong Kong was returned to Chinese rule in 1997.
The Joint Declaration is meant to guarantee the semi-autonomous status of the territory and the rights and freedoms of its residents for 50 years under a principle known as "one country, two systems".
China has rejected the British criticism and said the legislation was necessary and justified to protect Hong Kong from security threats such as foreign interference, subversion and secession.
A statement from the Chinese embassy in London to Sky News said that Hong Kong residents enjoy "unprecedented rights and freedom" and China upholds the rule of law.
"The British side has neither the right to supervise Hong Kong nor moral responsibility towards Hong Kong whatsoever," it said.
"As is in all countries, it is the central government that is responsible for upholding national security... Three million Hong Kong residents signed a petition in support of the law in just 10 days in the run-up to the enactment of the national security law.
"Since the law was enforced in Hong Kong, the local legal system has been improved, public order restored, and the sense of security of the residents and foreign nationals in Hong Kong increased. Stability, security, solidarity and development have become the consensus of the Hong Kong society.
"What the British side should do is to discard its colonial mentality, stop applying double standards… and respect the basic principle of non-interference in other countries' internal affairs in international relations."
You can hear more on this topic in Episode 7 of Into The Grey Zone.