Under complex excitation conditions, the leakage magnetic field caused by transformer winding will cause stray loss in the structural parts, which will cause local overheating, and serious operation accidents. Accurate measurement methods of stray loss of structural parts and effective measures to reduce the loss have become the focus of attention in the structural design and optimization of transformers. Based on TM21 experimental model, this paper presents a method to measure stray loss of transformer structure under complex excitation conditions by combining experimental measurement with accurate simulation calculation. This method can provide effective help for the design and optimization of transformer and smoothing reactor products.