Integration algorithm is used to calculate dynamic resistance of resistance spot welding. A model of "nugget diameter-dynamic resistance" is established according to the change rules of body resistance and contact resistance of sopt welding of dual-phase steel.DP600 dual-phase steel is selected as the test material, a self-developed spot welding quality prediction system is used to measure the dynamic resistance in spot welding process,and nugget diameter is measured by metallographic method,the values of dynamic resistances and nugget diameters in different current conditions are acquired,the model parameters of "nugget diameter-dynamic resistance" of spot welding of DP600 steel are obtained by the least square method. The test results show that the mean absolute error and relative error of nugget diameter predicted by this model is 0.13 mm and 2.28% respectively. This model has higher accuracy.