Yau Wang-tat had pleaded guilty to unlawful assembly, having initially been charged with the more serious offence of rioting.
In mitigation, Yau’s lawyer told the District Court that his client was only trying to help a protester who had been shot in the chest with a live round by a policeman, in Tsuen Wan on October 1, 2019.
The lawyer added that despite the stress of the court case, Yau had managed to complete his doctorate degree in physics at Polytechnic University.
But deputy judge Li Chi-ho said on Friday that these factors were nothing special.
Li said video footage showed protesters had planned the National Day violence, as they were seen throwing petrol bombs and rocks, and some had held a police officer on the ground.
The judge said although Yau was of good character, he decided on the 12-month prison term in light of the seriousness of the crime.
The teenager who was shot on the day in question, Tsang Chi-kin, was charged with rioting. He didn’t attend a court hearing last December, and a group based in the UK said at the time that he had gone into exile.