Quantitative identifying is a difficult problem of inspecting composite plates.In order to resolve it,in base of Lock-in thermographic NDT theory,studied problem how to apply the Lock-in thermographic NDT.Ascertain the relationship of phase shift and defect deepth,which make quantitative inspecting feasible.Phase shift will be the biggest with certain frequency.The rate of swing difference and heat flux is a constant.These provide academic basis to optimize parameter of thermal inspiritting signal and were great valuable to quantitative testing and evaluation of thermal wave NDT.