Abstract: The free expansion rate is an important indicator for determining the expansion characters of coal. Considering the disadvantages of high operational requirements and long time for indirect measurement of micrometers, the strain measurement method was used to test the expansion strain of coal water absorption. The research shows that: the strain measurement has the advantages of high reliability, high sensitivity, automatic data acquisition, etc., and can directly obtain the free expansion rate of coal; the whole process of coal water expansion deformation strain can be divided into initial compressive strain, rapid rise of tensile strain, and slow increase of tensile strain; axial tensile strain of coal specimen is about 1‰ larger than that of circumferential strain, and the difference is not large. The penetration rate of water along the bedding direction is fast, and bedding has great influence on water swelling of coal.