Abstract: In the spot welding process,the spot-welded joints of multilayer sheets were always available. The variation of the sheet thickness is obvious in the spot welding process,which makes the impedance fluctuation drastic in secondary circuit of welding power transformer. Thus,the spot welding heat also fluctuates evidently. In this case,the nugget quality cannot be guaranteed if welding heat is not regulated effectively in real time. By means of dynamic test and analysis of welding thermal process,the applicability of different monitoring methods was discussed for spot welding of multilayer low carbon steel sheets. The scientific basis therefore can be provided for manufacturing enterprises to select the exact monitoring method and to achieve the effective quality control.