The world No 3 came from one stroke down at the turn to card a three-under-par final round of 67 and win by two shots.
A birdie on the final hole helped Li seal second place ahead of England's Tom Lewis, whose five-under-par final round of 65 was enough to secure third.
Li had gone into the final 18 holes on a high after firing four eagles, including a record-breaking three eagle-twos, in his third-round 62.
But he fell away over the second half on Sunday, with three bogeys hampering his hopes of an impressive win.
Johnson recorded five birdies in his final round, including the last two holes as he finished on 19-under-par.
Lewis finished three shots off the lead after a blazing start to his final round, recording five consecutive birdies over the first five holes.
Yet his round was eclipsed by South Korea's Lee Min Woo, who finished a stroke further back after his second consecutive seven-under-par 63.
A four-under-par final round made Ian Poulter the next best-placed Briton in sixth, while Bradley Dredge and Ross Fisher were in a share of 13th.
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