Ilonen, who hasn't won on the European Tour since August 2007, turned in a 6-under 30 on his second nine, moving to 12-under 132 for the tournament.
He's three strokes ahead of Brett Rumford, who won in South Korea last week, and Kiradech Aphibarnrat, who each shot 67 to improve to 9-under for the tournament.
First-round leader Robert-Jan Dersken was 2-under for the second round and is alone in fourth at 8-under after 36 holes.
Ilonen opened on the second nine of the Binhai Lake Golf Course and birdied No. 10. He added birdies at 15 and 16 but really took off on the front side.
After making the turn in 3-under, Ilonen posted back-to-back birdies at 1 and 2 and again at 5 and 6. Closing birdies at Nos. 8 and 9 put him 4-under in his final five holes.