The girl was apprehended on Sunday in the Khanshin district of southern Helmand province, the Interior Ministry said in a statement.
She had been forced by her brother into committing the attack, a local police official said
"She was confused and scared at the time," the official said, adding that she was transferred to the provincial capital Lashkar Gah.
The Taliban have used kids as suicide bombers in the past. Last year, President Hamid Karzai pardoned a number of children who had attempted the attacks.