Abstract: The coal produced in southwest China fluctuates widely in ash content.Following an analysis of the problems encountered in determination of the ash of the coal using active ash monitor,the paper goes to introduce the working principle of the natural gamma-ray ash monitor as well as the result of test made with coal sample and field application at the washery of Panzhihua Coal Group and Daichiba Coal Washery.Practice shows the ash monitor can work with a performance well up to the requirement of the plants,and its widespread application in southwest China is well warranted.