China's No.1 Li, aged just 19 and a four-time winner on the PGA China Tour last year, put in a sparkling closing round of five-under 67 on home soil at Genzon Golf Club to take the clubhouse lead on 12 under.
But Kiradech, who had struggled on the greens all day, found his putting touch when it mattered and sank a 20-footer for eagle on the par-five 17th to share the lead.
The burly Thai had a chance to win outright on the 18th but his birdie attempt from 12 feet slid past the right of the cup and he signed for a level-par 72 and a total of 276, also 12 under.
The pair went head-to-head back up the 18th and Kiradech was faced with the exact same putt again for victory as crowd favourite Li missed his own long birdie attempt.
Second time round the 25-year-old nailed it, screaming with delight and relief after finally adding a second European Tour title to his 2013 Malaysian Open triumph.
"That was the best putt I ever made in my life," said an emotional Kiradech straight afterwards.
"Special word for my caddie. He said wait for the last two holes, find the fairways and we'll have our chance.
"It proved right and we got the job done."
England's Tommy Fleetwood was third on 11 under, having seen a 60-foot birdie putt on the final green that would have got him to the play-off miss by the width of a golf ball.
Seven players were tied for fourth a shot further back, including 2009 US PGA champion YE Yang of South Korea, who showed a welcome return to form after a terrible 2014 with a 69 to follow his six-under round of 66 on Saturday.
Earlier in the day Alvaro Quiros scorched to the best round of the week. The big-hitting Spaniard lit up the front nine with seven birdies, taking just 29 shots to the turn, two shy of the European Tour all-time record of 27.
He picked up four more on the back nine but gave shots back on the 12th and 18th for a nine-under round of 63 which propelled him into the top 15 at eight-under 280 for the week.
And Bubba Watson finally found his touch around the greens after struggling through his first three rounds.
The world No.4 closed with eight birdies in a round of seven-under par 65, jointly best of the week until Quiros surpassed it.
It gave the double Masters champion a five-under total of 283 for a share of 29th place.