In order to study the influence of welding flash on heat transfer performance of steel piston cooling gallery, the pistons with different structures are studied. In this paper, the finite element simulation calculation of the steel piston with the welding flash and the non-flash structure on the same model and the annular cavity section is carried out separately, and the amount of carbon deposited in the top of the steel piston and cooling gallery after 1000 hours test is measured. The comparative analysis finds that the existence of welding flash hinders the heat transfer of cooling gallery, resulting in an increase in the temperature of the inner surface of cooling gallery, which in turn increases the temperature of entire piston head. Carbon deposits in the top surface and cooling gallery of the steel piston, which affects the cooling effect.