Abstract: Before welding procedure for PG4 75 kg·m-1 on line heat treated steel rail, cracking and spalling damage were found on bottom surface of the steel rail. Specimens were sampled from the defective rails for test and analysis and it was found that all technical parameters such as chemical compositions, mechanical properties, microstructure and non-metallic inclusions met the requirements of relative standard. Based on characteristic of the bottom cracking damage, MSC·Marc software was used to simulate the stressed state when rails stacking. The results showed that when some rails were non-level stacked, excessive stress would apply on one side of the rail bottom, which resulted in the bottom cracking damage finally.