The accuracy of ash detection is an important basis for ensuring coal quality evaluation, coal settlement and industrial production control. The particle size of sample preparation is the key controllable factor affecting the stability of ash detection. Anthracite has a dense structure and high hardness, and inorganic minerals are finely embedded in the coal body. Compared with bituminous coal and lignite, the particle size of sample preparation is more sensitive to the ash detection results. If the particle size is too coarse or too fine, it will cause systematic detection deviation. In order to explore the response difference of anthracite coal samples to particle size deviation, this paper selects anthracite as the test object, sets multi-gradient particle size conditions, and uses slow ashing method to carry out comparative tests. It can provide technical support for optimizing the preparation process of coal samples and improving the reliability of ash detection data.