Park finished 36 holes of the year's first major event on the LPGA Tour at 7-under 137, giving her a one-shot advantage over Lizette Salas and a two-shot cushion over Giulia Sergas and Caroline Hedwall.
Park made three straight birdies beginning at the 11th hole, then followed that stretch with her only bogey of the day at the par-3 14th.
Among those tied for fifth place at 140 was first-round leader Jodi Ewart Shadoff, who settled for an even-par 72.
Michelle Wie, trying to get her once-promising career back on track, jumped into contention with a 3-under 69 that put her in a tie for eighth at 141.
Wie has finished no better than 45th this year and has missed the cut in three of her five tournaments.
She has adopted a radical putting stance in which she bends dramatically the waist so that the upper part of her body is parallel to the ground. Although she missed a series of short putts, she had five birdies in her round.