A photo posted by Chinese media online showed several women and children sitting or lying on the ground, some bleeding.
The blast occurred at the gate of the kindergarten as children were leaving the school in the afternoon, according to the Xinhua news agency.
An official at the police station in Fengxian county in Jiangsu province said the cause of the explosion was under investigation.
The images circulating online showed more than a dozen people outside a building, many lying on the concrete and some appearing to be unconscious, including a small child. The force of the blast tore the clothes off some people.