With Q345D used by railway vehicles as experimental material,spot welding experiments were carried out under the premise of fixed spot welding time. By analyzing the dynamic resistance curves of spot welding process,the characteristic quantities which can characterize the nugget growth process of spot welding were found out,and the prediction model of spot welding quality was obtained with the characteristic quantities as variables. Spot welding experiments were carried out with the same welding time of the prediction model,and the tensile strength obtained by the prediction model of spot welding quality was compared with the experimental tensile strength. The average error of the tensile strength of spot welding was 3.3%,and the maximum error was 6.05%.