The workers are trapped over one kilometre from the entrance. Rescue workers and pump trucks are still removing rubble and water from the underpass which runs underneath a reservoir.
Yan Dawu, deputy general manager of the railway group, told reporters the construction workers heard an “abnormal noise” on Thursday at around 3.30am (7.30pm GMT) and decided to evacuate the tunnel.
However, water began flooding in before everyone could get out.
Mr Yan said the workers are believed to be stranded some 1,160 metres away from the tunnel’s entrance.
Rescue efforts have been hampered by the large influx of water at the tunnel face.
Around 15,400 cubic metres of water are being drained from the tunnel hourly by five mobile drainage trucks.
The city’s vice-mayor said the point of flooding has been blocked.
Another 20 pump trucks are on standby in the surrounding area, he said.
The tunnel forms part of an urban plan to build new highways in the city connected to the enormous bridge linking Hong Kong, Macau and Zhuhai, according to the local government website.