Abstract: In order to conduct repair design of defect pipelines more rationally, it is necessary to analyze the applicability and accuracy of repair design methods. In “Repair of Pressure Equipment and Piping (ASME PCC-2-2011)”, three repair design methods are put forward based on three assumptions, i.e., “underlying substrate does not yield”, “underlying substrate yields” and “repair laminate allowable strains”. In this paper, the correlations between three repair thickness calculation methods and the design pressure were investigated based on mechanical analysis. Then, crack-like defect with depth/thickness ratio of 70% and semi-elliptic defect were designed and prepared. Finally, the repair thickness of composite materials was calculated under different design pressures by means of three design methods, and the sensitivity of repair thickness to design pressure was analyzed. It is indicated that the repair method based on the assumption of “underlying substrate does not yield” is extremely sensitive to design pressure, the method based on “repair laminate allowable strains” is relatively conservative and the one based on “underlying substrate yields” is the most applicable to repair design. (4 Figures, 1 Table, 20 References)