The 29-year-old, who stormed to gold in the 100 and 200 metres at the 2004 and 2008 Games, became the first Japanese swimmer to qualify for four Olympics by breaking his own national record in the 100 earlier this week.
Kitajima clocked two minutes 8.00 seconds to win the 200 at Tokyo's Tatsumi Pool, fighting off a determined last 50 from Ryo Tateishi, who touched in 2:08.17.
"I can't speak!" gasped Kitajima, struggling to regain breath after a rousing battle with Tateishi, who also qualified for London as runner-up in the 100 and 200.
"I was right on my limit at the end as you saw," said Kitajima. "But that's the sort of race I live for and to prove I can still do it gives me confidence."
Both men beat Hungarian Daniel Gyurta's world championship winning time of 2:08.41 in Shanghai last year.
Kitajima's national record stands at 2:07.51 while Australia's Christian Sprenger holds the world record at 2:07.31.
"It definitely won't be easy (to win double gold in London)," said Kitajima, who flirted with retirement after blowing away his rivals at the 2008 Beijing Olympics.
"But the fire is still there to keep improving and going quicker and to prove myself again at the Olympics. It's still the stuff of dreams to prove yourself at an Olympics."