Landslides triggered by the disaster have blocked dozens of rivers in Sichuan province, leading to the formation of many new lakes, some of which have already engulfed villages.
Officials are particularly worried about the Tangjiashan lake, where the water level has risen rapidly to within 26 metres of the fragile dam's lip.
Landslide debris is holding back about 130 million cubic metres of water at the site in Beichuan county.
Emergency workers aim to evacuate the 80,000 people by midnight (1600 GMT) on Tuesday.
Troops are using earth-moving equipment as they try to ease pressure on the lake's banks. They are also preparing to use explosives.
More than five million people remain homeless following the 7.9 magnitude quake and the area is still being rocked by sizeable aftershocks.
Six people died and 300,000 more homes were wrecked by a strong tremor on Sunday.
Storms have been forecast for the region, potentially compounding problems as rain water drains into the lakes.
Officials have said that reconstruction work in the area is set to take at least three years.
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