The snowslide occurred Tuesday evening at the exit of a tunnel on a highway in southeastern Tibet, trapping a number of people in their vehicles.
China's Ministry of Emergency Management sent a disaster response team to the region the following day.
State media reported Friday that authorities had now placed the death toll at 20, with eight people still missing.
"At present, the rescue work is still ongoing," state broadcaster CCTV reported, citing rescue authorities.
The Duoxiongla Tunnel was shrouded in cloudy weather but it was not hindering rescue work, CCTV added.
Avalanches are common in the Himalayas, home to the world's highest mountains.
In October at least 26 people died when a mountaineering expedition was caught in an avalanche on Mount Draupadi ka Danda-II in India's northern state of Uttarakhand.