PC Peter Hodgkinson discharged the weapon while Andrew Pimlott, 32, was holding a lit match at his parents' home in Plymouth in April 2013.
Mr Pimlott caught fire and died from his injuries five days later.
The officer said he only fired because he feared Mr Pimlott was about to send himself up in flames.
The inquest heard two police officers went to the home of Mr Pimlott's parents after his father called 999.
His father Kelvin said his son had a can of petrol and was threatening to set fire to the building.
The inquest previously heard the dead man had been served with a restraining order preventing him contacting his parents or going near their home.
After PC Peter Hodgkinson and PC David Beer arrived at the scene, they saw Mr Pimlott, who was in the back garden, douse himself in petrol from a can and strike a match.
"I deployed the Taser to reduce the risk the of harm to Mr Pimlott," said PC Hodgkinson.
"I tried to engage with him to put down the can. I believe he was going to set himself alight so I fired the Taser."
PC Beer said: "I heard the Taser sound being activated and Mr Pimlott went up in flames."
Independent fire investigator Steven Andrews said he made a "very strong finding" that Mr Pimlott was set on fire by the Taser.
The ignition point was on the belly where the Taser hit.
"The evidence does not support that he was set alight by the match," he said.
The inquest continues.
A police officer said he hoped a shot from a Taser stun gun would prevent a man soaked in petrol setting himself alight, an inquest has heard.