Hot spot stress (HSS) approach is more applicable for fatigue evaluation, because it is more closer to the real stress at the welded notch than nominal stress.This paper studies a typical welded joint, fillet welded joint, which is used widely in high-speed car. Hot spot stress concentration factor is calculated under different nominal stress and loading methods. The S-N curves belong to nominal stress and hot spot stress were analysized. The result show that the difference between the S-N curves under different load style can be reduced greatly by hot spot stress. The hot spot stress characteristic variable was put forward, which is used to describe the influence from the welding size of the joint.