The design and calculation on work clearance and relative interference amount were studied in order to make non-backpressure piston rings work normally and to raise its life. The formula of thermal expansion work clearance was derived by thermal deformation analysis. Radial amount of relative interference was designed by mechanics analysis,which can ensure that piston ring has not backpressure. The formulas in the paper can help design,processing and assembling for non-backpressure piston rings.