The accident took place at a power plant in Fengcheng where a cooling tower was under construction.
One person is still missing, state media say. Jiangxi province's fire services said 32 fire trucks and 212 soldiers had been deployed.
Several people were injured. Industrial accidents are common in China prompting calls for better safety standards.
Photos posted by Chinese media showed iron pipes and concrete slabs lying on the ground inside the large cooling tower.
According to Jiangxi Daily, two 168m cooling towers were being built at the site as part of a project to add two 1,000-megawatt coal-fired power units to the power plant.
The project will cost 7.67bn yuan ($1.11bn; £0.89bn) .