The 600MPa class steel for automotive frame have developed on Flexible Thin Slab Rolling line, and then fatigue performance of this steel was studied by using up-and-down method. It was found that lgS-lgN fitting was better than S-N fitting, and P-S-N curves under different survival rates were calculated. The fatigue limit obtained through accurate calculation is 287.1MPa .The fatigue fracture was analyzed and observed, and it was found that the fatigue fracture was ductile fracture and the inclusion was the original of the crack source. So the developed steel can meet with the operating requirement by combined with performance indicators and loading trial.