To resolve the conflict among strength, stiffness, and lightweight requirements in T-beam formwork design with multiple parameters, this study proposes a multi-objective optimization framework. A representative sub-section template was analyzed using finite element analysis (FEA) to evaluate stress and deformation. Design variables included channel steel spacing, stiffener spacing, and panel thickness. The NSGA-III algorithm was employed to minimize maximum stress, deformation, and mass while satisfying mechanical constraints. Optimization results show that the sub-section template achieved a 38.66% mass reduction: panel thickness decreased from 6 mm to 3.6 mm, with unevenly spaced channel steels and stiffeners. Validation on the full-scale T-beam formwork demonstrated a 19.44% lightweight improvement while meeting stiffness and strength standards. This method effectively balances performance and lightweight goals, offering practical value for large-scale structural optimization.