Magistrate Jeffrey Sze had convicted 21-year-old Zhao Hong-shing of assaulting the officer in the Tin Shui Wai police station on January 2.
Sze then ordered Zhao be jailed for eight months, citing the seriousness of the offence.
But the defence lawyer pointed out that imprisonment for the offence that Zhao had been charged with under the Police Force Ordinance is capped at six months.
After a review, Sze said the case is not the most serious on the spectrum, and changed the sentence to four months of imprisonment.
The magistrate had earlier ruled that Zhao was an unreliable witness, saying he had offered different accounts of the incident. He had also noted that the police officer’s injury was consistent with a medical report.