She told how her brother, reportedly a Taliban commander, had forced her to wear the vest and ordered her to detonate herself at a police checkpoint.
"I was tired of my stepmother. My brother told me to wear the black vest, go to the police checkpoint and press the button," she told reporters.
"I went past a river and decided to drop the vest. My brother fled and police arrested me."
There are conflicting accounts of the incident, with some officials saying she was wearing the vest when she was arrested and others saying no vest had been recovered.
The Afghanistan-based Tolo TV news channel said the girl was unable to operate the button to detonate the explosives.
The interior ministry said that her brother was a Taliban commander who coerced her into putting on the vest and walking towards the police post in Khanashin district in the southern heartlands of the insurgency.
Taliban militants have been accused in the past of using young boys to plant roadside bombs and act as suicide bombers.
The insurgents, who made no comment on the latest incident, have always denied using children to launch attacks.
The Taliban has been fighting against the US-backed government since it was ousted from power in late 2001 after the 9/11 attacks in the United States.