As a sophomore, he drew national attention at the President's Cup National High School Baseball Championship held in April 2007, where his team finished runner-up and An won batting, home runs and RBI titles.

He became the first rookie player to earn the All-Star Best 10 honors since Lee Byung-Kyu and Jin Kab-Yong in 1997.

An finished his rookie season with a batting average of .235 (44th in the league) and 14 home runs.

Although An was the only rookie player to be eligible for the batting title in the 2009 season, having 436 plate appearances in 123 games, he failed to make the Rookie of the Year title, losing out to saves champion Lee Yong-Chan of the Doosan Bears.

In the tourney, he batted .333 (10-for-30) with 8 RBIs and 8 runs, playing in all 8 games as a second and third baseman, and was named Best Defensive Player.