Starting in 2010, carefully selected batches have been released into the reserve.
"Now there are 27, 16 females and 11 males," says Sun. "We even registered the birth of a foal in July 2011, a new success in our reintroduction process."
But very few animals can endure an environment as hard and dry as Gansu's desert steppe.
Przewalski's horses require daily access to water that is within a 30-kilometre (20-mile) range and does not freeze in winter. They also need 22 pounds of dry food per day, relatively close to the water.
In a region that receives less than 1.5 inches of rain per year, many of these conditions could become problems, says Sun.
The reserve is taking back-up measures to improve the horses' chances.
"We have increased the water supply by expanding 10 wells. At a later point we are thinking of bringing water from the river.
"In winter we have to break the ice so that the horses can drink."
The horses eat grasses and certain plant species, says reserve employee Lu Shengrong, but when vegetation becomes sparse in winter-time, they will be fed dry alfalfa, straw, black beans and corn.