The 19-year-old, who won the World Cup in London which doubled as the Olympic test event in April, showed her liking for the Royal Artillery Barracks by setting an Olympic record in qualifying.
Then in the final, she rattled off a five-shot final round of 51.8, including a perfect bullseye of 10.9, to leapfrog Beijing winner Chen at the death.
Olena Kostevych, of Ukraine, took the bronze medal.
Kim was aware she had slipped to second and revealed her determination to snatch the gold with her final effort.
'For the first time in 15 shots, when I came down to second and Chen went up to first, I thought "I really don't want silver, I want gold",' she said.
'It's not a very nice thing to say but I just desperately wanted the gold.'