The workers shouted through the air vent, stating that there were safe, state-run Xinhua news agency reported.
They confirmed that the vent was working properly in the afternoon by knocking on a 27-meter-long iron tube, according to local work safety authorities.
Rescuers were informed that 21 workers were trapped after a 30-metre long section of the expressway tunnel in Longyan City collapsed.
Debris, estimated at 5,000 cubic metres, have blocked the tunnel which was being expanded to accommodate the expressway linking Xiamen City of Fujian and Chengdu, capital of country's southwest Sichuan Province, state-run Xinhua news agency reported.