Abstract: The tensile testing of I-shaped structural sealant specimens is used to measure the initial stiffness modulus simulating the actual stress state of sealant in glass curtain walls. However, current testing approaches using specimens with uniform width-to-thickness ratio cannot reflect the boundary constraint effects of different width-to-thickness ratios on the initial stiffness modulus. In this study, a novel calculation method is proposed to calculate the initial stiffness modulus of structural sealants under boundary constraint effects by use of the unconfined initial elastic modulus. Five dumbbell-shaped specimens were tested to measure unconfined initial elastic modulus tests. 45 I-shaped specimens with different width-to-thickness ratios were tested to measure the initial stiffness modulus. The testing results showed that the initial stiffness modulus of the structural sealant increases linearly with the increase of the width-to-thickness ratio. After finite element models were validated by comparing the experimental data and numerical results, the influence of sealant thickness and width-to-thickness ratio on the initial stiffness modulus was investigated by a series of numerical analyses. Consequently, a calculation method was established to calculate the initial stiffness modulus by use of the initial elastic modulus. The applicability and accuracy of the method proposed are validated by comparing the effects of different initial stiffness modulus determination methods on glass panel deformation calculations, using a practical glass curtain wall project in Qingdao as a case study.