The grain size of rail flash-butt welding joints affects the wear resistance and mechanical properties of the welded joints.Therefore,for U75 V rails used in high-speed and heavy-duty seamless railways,the average grain size in the temperature of 800～1 100 ℃ and different holding times is measured by an austempering test. The results show that when the temperature is 800～950 ℃,the growth rate of the grains is slow,the average growth rate is about 0.4 μm/min,and the growth speed of the grains accelerates at the temperature of 950～1 100 ℃,and the average growth rate increases and is approximately 1.6 μm/min. A mathematical model of the grain growth of U75 V rail steel under isothermal conditions is established. In addition,the finite element analysis software Abaqus is used to establish the rail flash-butt welding model and the influence of different temperature fields on the grain size of the joint is analyzed,and the simulation results are verified by experiments.