To investigate surface stress distribution of rectangle piston ring, method of increment loading based on energy variational principle is used to ascertain contact state and solve elastic contact problems by iteration. Calculation results show that the central section of the first sealing surface is subjected to lower stress while two sides higher stress. The theoretical analysis and experimental results indicate that appearance of stress peak on surface is related to movement direction of piston ring. Relatively critical wear regions of piston ring are consistent with finite element analysis results. Wear width of both sides is 0.2mm more or less. The section of maximal stress appears at position near ring groove of the first sealing area.