Based on the theory of fracture mechanics, combined with rail bottom measurement, the model of rail bottom crack expansion has been established and indicators such as critical crack size, average crack extension rate as well as remaining life has been quantified. In terms of the rail material, residual stress, temperature stress and other factors, work conditions are respectively designed to explore the influences of rail bottom crack expansion. It is found that there exists positive correlation between the fracture toughness of rail material and the remaining life. The remaining life of U95Cr H is less than U78CrV H, which is less than U75V H. Take U78CrV H, used in the line under test, as an example, when the residual stress grows from 0 to 300 MPa, the critical crack size grows by about 2.5% and the remaining life decreases by about 6%; and when the residual stress is 200 MPa, increasing the temperature stress from 0 to 200MPa, the critical crack size does not change much and is within 1%.