Park eagled the sixth hole and had two birdies on the front nine before coming home in 29 shots, with six birdies in nine holes to complete an 11-under-par round.
That left her one shot ahead of American Angela Stanford, who had seven birdies in a bogey-free 64 to nudge ahead of Catriona Matthew.
The Scot had led jointly with Stanford after the first round and by a clear three shots going into today's play, but could only manage a 68 to end the day two shots behind Park in third outright.
Korean Meena Lee was fourth on 17 under with Swede Anna Nordqvist another two strokes back and Gerina Piller, Amy Yang and Austin Ernst on 14 under.
World number one Inbee Park appears to be out of the picture at 13 under after five birdies and two bogeys gave her a 68.