Abstract: In response to significant data fluctuations and measurement errors in dual−energy gamma−ray online ash monitors caused by variations in mass thickness, this paper proposes an ash measurement device and method based on low−energy gamma rays with constant coal seam thickness. The device maintains the optimal distance between the coal seam and the detector surface and ensures a constant thickness of the coal sample. It simultaneously achieves refined particle sizing and a narrower particle size dis-tribution, ensuring a flat coal flow surface, stable compactness, and consistent moisture content. This establishes an optimal measurement environment for low−energy gamma−ray ash detection. Experimental results with heavy medium concentrate coal from the Sunyuan−Tongting and Xutuan−Linfei washing plants showed that the standard deviations of the measured ash content were significantly reduced to 0.337 and 0.206, respectively, compared to the corresponding laboratory−analyzed standard deviations of 0.442 and 0.353. Using the mean absolute error between measured and laboratory values as the evaluation metric, the average errors for the two coal samples were 0.32% and 0.31%, respectively, both within the permissible error limit of 0.50%. Following parameter calibration and experimental verification, this low−energy gamma−ray constant−thickness measurement method demonstrates high stability and accuracy in ash content detection.