A Perth policeman accused of grabbing a homeless man by the throat during a strip search and "slamming" him to the ground has been acquitted.

Sergeant Lachlan Nairn Boath went on trial last month in the Perth Magistrates Court accused of assaulting Clinton Nicholson in June 2012 at the Perth watch house, where he was the officer in charge.

Mr Nicholson testified he was a homeless drug user at the time and admitted being "a little mouthy" to police before he was "charged" at by Boath and left gasping for breath.

Magistrate Geoff Lawrence said the prosecution's case was "found wanting" and failed to prove Sgt Boath's guilt beyond reasonable doubt.

It was established the detainee had been "rushed" and his breathing had been impinged, but how the constraint occurred had not been established.

Sgt Boath said the case had put him under intense pressure and he felt relieved by the magistrate's decision.

He said the charge should not have been laid in the first place.

"When you're attempting to do your job and this is the reward that you get for it, I'm sure you can imagine it can be quite stressful - makes you wonder whether it's worth the efforts," he told reporters outside court on Friday.

"I think I'm going to have to reassess that after today's result.

"When you're being brought to task on use of force issues, it certainly makes you think twice about utilising use of force ... options when you think this is a likely result."

Sgt Boath said he had questioned the use of restraint several times since the incident and had been left with a shoulder injury because he held back while dealing with a detainee on one occasion.

WA Police Union senior vice president Brandon Shortland said it was another example of a Crime and Corruption prosecution against a police officer failing and he hoped the new leadership of former Supreme Court judge John McKechnie meant the CCC would pursue more serious matters.

"This man had done nothing wrong - he was simply doing his job as a police officer, which is a difficult and dangerous job under very trying circumstances," he said.

Sgt Boath, who had been on full duties throughout, had faced "minor" internal disciplinary action "for policy reasons", Mr Shortland said.

The prosecution was ordered to pay costs of $16,368.