The Independent Police Complaints Commission (IPCC) upheld a complaint by Kyle McArdle, who was arrested for urinating in an alley in 2009.
Mr McArdle was later cleared by magistrates of assaulting two PCs.
Merseyside Police said it welcomed the watchdog's comments and it was now reviewing its use of Tasers.
PCs Simon Jones and Joanne Kelly were dismissed for gross misconduct after a disciplinary hearing while a third officer, a sergeant, had already been dismissed from the force for an unrelated matter.
'Barbs removed'
“I feel that justice has now been done and I can get on with my life,” said Mr McArdle.
The 26-year-old was arrested in December 2009 when he was spotted urinating in an alley off Elliot Street.
He was put into a van and shot five times with a Taser, three times with the weapon pressed against his chest, leg and upper abdomen.
The arresting officers claimed he was violent and Taser was needed to restrain him.
Taser barbs were also removed from his chest by an officer despite guidelines which say they should normally be removed by a medical professional.
The officer claimed he feared Mr McArdle would remove them himself to use as weapons.
Mr McArdle complained that the repeated use of a Taser in the confined space of a police van was disproportionate.
The IPCC upheld his subsequent appeal and recommended Merseyside re-investigate, considering if the Taser use would be justified had the victim been lawfully arrested.
Merseyside Police's leading Taser instructor found that the Taser use was “necessary, proportionate, reasonable and in line with the officers' training”.
The watchdog upheld a second appeal, finding that the officers should have been served with notices for gross misconduct and interviewed under caution.
IPCC Commissioner James Dipple-Johnstone said: “While we welcome the robust action eventually taken by the force in response to our appeal findings it is a concern that Merseyside's lead Taser instructor lacked objectivity and presented as fact the officers' version of events without challenge.”
