The activist, Zhao Lianhai, was sentenced to two and a half years in prison for "disturbing social order," Amnesty International said.
Zhao founded the support group and Web site "Kidney Stone Babies" after his own child was sickened by tainted milk.
The milk was found to contain the toxic industrial chemical melamine so it could pass government tests for protein.
Amnesty International's deputy director for Asia said Zhao should never have been arrested.
"We are appalled that the authorities have imprisoned a man the Chinese public rightly views as a protector of children, not a criminal," Catherine Baber said.