Abstract: In order to reduce the operation time for thermite welding repair of the railway line,an improved preheating technology to reduce preheating time was developed.The microstructure,hardness,mechanical properties and fracture mechanism of thermite welded rail steel joints treated at different preheating times (2.5,3.5,4.5,and 5.5 min) were studied.It has been found that the microstructure of steel joints which were preheated for the above mentioned times was all fine ferrite-pearlite.The hardness at the center of the joints was the lowest and increased with the distance from center,slightly increasd with reduction of the preheating time.The σb of the railhead along the welding line was the largest,σb the railbase was slightly lower,and σb of the railwaist was the lowest.The σ0.2 and σb declined with the increase of preheating time,however,there was little influence on ductility.The fracture mechanism was brittle and the fracture was cleavage type.