Abstract: Thermal excitation of coal seams by high-temperature steam is a highly promising technology to increase gas production. Steam permeability is a key parameter characterizing the injection capacity of thermal fluids. However, the seepage law of steam in coal and its evolution mechanism are still unknown, and the exploration of the above issues is of great scientific significance for gas production enhancement by heat injection. The experiments on seepage and thermal strain of high-temperature steam injected into coal were carried out using the steady-state method, and the dynamic change laws of steam permeability and thermal strain of the coal with time were investigated. The theories of Kelvin capillary condensation, plug flow and thermal stress were used to investigate the evolution mechanisms of condensed phase change, pulsating seepage of steam and thermal strain of coal, respectively. The results indicated that during the process of high-temperature steam injection into coal, with the extension of the injection heat time, the liquid-measured permeability of steam shows an intermittent pulsation law. With the increase of steam temperature, the pulsation peak decreases, the periods are shortened, and the oscillation is more intense. During the heat injection process, the radial and volumetric strains of the coal show 2~3 expansion stages. When the steam temperatures are low, the axial strains are compressive, and when the temperatures are high, the axial strains turn to expansion. The study demonstrates that the equilibrium pressure of steam in the micro-pores of coal is less than the saturated vapor pressure in the large space, and the smaller the pore diameter, the lower the pressure required for steam condensation, and the easier steam condense. The gas-liquid plug flow induced by steam in coal is the main mechanism causing intermittent pulsation of permeability. In addition, the high-temperature steam has superimposed inward and outward expansion influence effects on the permeability of coal, leading to a decrease in the permeability of large pores and an increase in that of small pores in the matrix. During the steam injection process, the rapid expansion strains in the early stage are mainly controlled by the pore pressure, and the slow expansion strain in the middle and late stages by the thermal strain. The results provide factual basis and theoretical reference for the practice and numerical simulation of steam thermal recovery gas.