The isothermal local loading forming technology provides a feasible way to form Ti-alloy large-scale rib-web components in aerospace and aviation fields. However, the local loading process forming limit is restricted by forming defects in the transitional region. In this work, the feasibility of controlling forming defects and improving the process forming limit by adjusting die parameters is explored through finite element (FE) simulation. It is found that the common cavum and folding defects in the transitional region are significantly influenced by the fillet radii of left rib and middle rib, respectively. The cavum and folding defects can be effectively controlled by increasing the fillet radii of left rib and middle rib, respectively. The process forming limits considering forming defects in the transitional region are determined by the stepwise searching method under various die parameters. Moreover, the relationship between the process forming limit and die parameters is developed through the response surface methodology (RSM). The developed RSM models suggest that increasing the fillet radii of left and middle ribs is effective to improve the process forming limit during local loading forming of rib-web components. The results will provide technical basis for the design of die parameters and the reduction amount, which is of great importance to control forming defects and improve the process forming limit in local loading forming of Ti-alloy large-scale rib-web components.