Abstract: In order to improve the performance of rail aluminothermic welding joint, the influence of the rail gap width on preheating time and joint performance was investigated. The results show that the rail gap width had great effect on preheating time and static bending strength of the joint during welding. The larger the rail gap width, the longer the preheating time. When the rail gap width was 20-22 mm, the preheating affect of the rail base was bad. It was easy to form lack of fusion, the static bending load could not reach the standard; when the width was 32-40 mm, it was easy to form micro porosity in the middle of welding joint, and the static bending load was lower than the standard. When the rail gap width was 24-30 mm, the load could meet the TB/T 1632.3-2005 standard.