"The miscalculation was due in part to the chaos in the aftermath of the quake," Xinhua cited Cao Jianfang, a deputy provincial governor, as saying.
The US Geological Survey said the earthquake was of a moderate 5.7 magnitude, and that it struck at 7:19pm local time (1119 GMT) on Thursday at a shallow depth of 10km, with the epicentre located 98km east-northeast of the city of Dali.
Xinhua put the magnitude at 6.0 on the Richter scale, and said it was centred in the province's Yaoan county.
The earthquake was followed by eight aftershocks, Xinhua said.
Yunnan's civil affairs department was sending 4,500 tents, 3,000 quilts and other relief materials to Yaoan, Xinhua said.
Hundreds of police were dispatched to the disaster zone, it said.
Al Jazeera's Tony Cheng, reporting from Beijing, said the area, being quite remote and fairly mountainous, would make it hard to reach and difficult to get information from.
He said that most of the destroyed homes would have belonged to ethnic Tibetans who live in the region.
Xinhua said medicine and food were also being distributed to people, many of whom had gathered outdoors for fear of more aftershocks.
Yunnan is a quake-prone, mountainous region that lies on China's southern border with Thailand and Myanmar.
It also borders Sichuan province, where a magnitude 7.9 earthquake a little more than a year ago left nearly 90,000 people dead or missing.
About 375,000 people were injured in the May 12 quake, which left five million homeless and up to 1.5 million people displaced.
The deadliest quake to strike China in over 30 years flattened entire cities and towns, and destroyed hospitals, homes, buildings and factories in nearly 50,000 villages.
Thousands of children were buried alive when some 7,000 schools collapsed in the quake.
Southwest China sits on two tectonic plates - the Indian and Asian plates - whose constant collision created the Himalayan range and the Tibetan plateau.