Abstract: Fine detection of thin coal area in coal face is of great significance for safe and efficient mining and roadway reconstruction. Firstly, the principle of fine detection method of thin coal area in extremely unstable coal seam based on in-seam wave transmission method is analyzed. Then, a typical three-layer geological model with different coal seam thickness is established, and the theoretical calculation is carried out based on the dispersion equation. The results show that the main frequency of in-seam wave will move to the low frequency with the increase of coal seam thickness; the correspondence between in-seam wave frequency and coal thickness and wave velocity can be used as the theoretical basis for in-seam wave transmission method to detect thin coal regions.