Abstract: Following an introduction to the problems encountered in the use of active ash monitor for online measurement of clean coal ash,and the working principle of the passive version,the paper goes to make an analysis of the feasibility of practical application of the latter for clean coal ash measurement,and an evaluation of the result of application of the monitor for clean coal ash measurement at Guobei Coal Preparation Plant. Field application of the monitor shows that the measurements are not affected by the presence of residual medium in the heavy medium separation system,and its measuring accuracy is far better than that specified by the standard EJ/T1078-1998 for 《γ radiation Coal Ash Monitor》.