Abstract: Aiming at research on fatigue life of girth weld of aero-engine combustor casing mounting seat, a design method of biaxial tensile simulating specimen for welding structure of mounting seat was proposed. Firstly, through analyzing load conditions and geometric characteristics of mounting seat, the corresponding design criteria and geometric equivalent analysis were given, and prototype of the simulating specimen was designed at the same time. Then, according to optimization objective, an optimization mathematical model was proposed, and through finite element calculation method, stress change law of simulating specimen was discussed under three dimensional variables, including diameter D₁ of central test area, wedge angle ω of clamping arm and diameter D₂ of through-hole of mounting seat, and then, the final size of simulating specimen was determined. Finally, biaxial tensile fatigue test was carried out with simulating specimen. The test results showed that crack source appeared in the weld of simulating specimen and expanded to both sides along the weld, which proved rationality of the design.