Abstract: Uncertainty welding quality in spot welding dual phase high strength steel encumbered the steel being largely applied in auto-body manufacturing. Firstly, feasibility of indentation depth measuring online by servo gun was studied, which was shown that servo gun precision can meet indentation depth online measuring requirement. Secondly, indentation depth changing characteristic with welding numbers was studied, which was shown that changing speed of indentation depth was higher at earlier welding stage and became stabilization at later stage. Lastly, relationship between indentation depth and welding quality was studied, which was shown that indentation depth at 15%~25% range can meet welding quality requirement. Online evaluation welding quality based on servo gun measuring indentation depth can solve the question that inspecting method of traditional welding quality was lagging and not online.