Through the analysis of the microstructure, hardness test,pulsating tensile fatigue test and the analysis of fatigue fracture morphology,this paper studied the microstructure and fatigue performance of resistance spot welding joint of 1.5 mm Q310 and 3 mm Q345. The results show that,the microstructure of Q310 and Q345 are similar,the base organization is pro-eutectoid ferrite and pearlite,the microstructure of HAZ is a few ferrite and pearlite. The microstructure of nugget area is lath martensite and bainite. From the nugget area to the base material,the hardness value distribution is in a step downward trend,the hardness value of nugget is the highest of the whole joint. The fatigue fracture is located in the nugget area. And the type of fatigue fracture is mixed with micropore and cleavage,the main cause of fatigue fracture is stress concentration.