A West Midlands Police disciplinary hearing was told Inspector Matthew Downs also swore at a colleague and turned a cell light off after the 17-year-old detainee said he was scared of the dark.
The hearing at the force's HQ on Tuesday heard the youth, referred to as AB, was taken to Oldbury's custody suite after being "red-dotted" with a Taser when he threatened someone with a machete while drunk in Walsall.
Giving evidence after CCTV footage of the incident in the cell was shown to a misconduct panel, 49-year-old Downs said he had reacted instinctively after a threat was made to other officers.
Outlining the facts of the allegations admitted by the officer, presenting counsel Andrew Waters said: "On the eighth of February last year, in the early hours, a 999 call was made to police reporting that a drunken male was making threats to an individual with a machete.
"Officers attended the property and AB was, to use police phraseology, red-dotted with a Taser.
"He was arrested in reasonably peaceful circumstances and he was taken by two Pcs to Oldbury custody suite."
Custody block inspector Downs, who admitted gross misconduct, had watched other officers carry out standard search procedures, the panel heard, before deciding to become involved.
Mr Waters continued: "He entered the cell and used unnecessary and gratuitous force on AB - in particular in picking him up... pushing him around.
"In addition he was abusive to AB in two instances. He shouted and swore at AB and, when AB started crying, he mocked him for doing so.
"As he left the cell, he asked AB if he was afraid of the dark and, when AB said he was, he said 'Good' and turned the light off."
In his evidence to the panel, Downs, who has more than 22 years of police service, said the detainee had behaved very aggressively.
"I just wanted to tell him to behave himself," Downs said, acknowledging he had made an error in trying to lift the youth to his feet.
"As the officers were leaving the cell, AB made a threat. It was along the lines of 'You see what happens now', which I reacted to instinctively," Downs said.
Asked how he felt after viewing the CCTV footage, Downs replied: "I was disappointed with myself. It was a mistake, I shouldn't have done it.
"I have let myself down."
The officer's QC, Adrian Keeling, urged the panel to impose a final written warning in relation to the conduct towards the youth, who did not make a complaint about the incident.
Mr Keeling submitted: "The culpability and harm is low. It's a single incident and it's in no way targeted or planned - it arises out of a spontaneous set of circumstances."
Asked if he would agree to be reduced in rank if the panel decided to impose it as a sanction, Downs answered: "Absolutely. I am proud to serve."
Ordering that Downs be reduced in the ranks from inspector to police sergeant, panel chairman Nick Hawkins said a final written warning would not serve to maintain public confidence in policing.
Imposing the higher sanction of reduction in the ranks, Mr Hawkins accepted that Downs did not know the youth was a juvenile.
Mr Hawkins added: "We accept that this was a single incident that lasted a few seconds and that there was no malign intent to your actions."