Parameter-based damping ratio sensitivity (DRS) is usually used for designing multi-loop complex control systems in grid-forming (GFM) inverters. However, it is found that this method will be affected not only by the interactions among parameters in a controller, but also by the change of the parameter itself, which makes the parameter design based on the parameter-based DRS unreliable. To address these issues, each controller is considered as a whole, which avoids the interactions among controller parameters in a controller. Then, the controller-based determinant sensitivity (DS) that calculates the sensitivity of each controller to system stability is proposed for designing the control system, which will not be affected by interactions among parameters since the sensitivity is calculated at the controller level. Then, combined with the proposed solution, a controller shaping method is also presented, which can improve system stability by shaping the controller. Finally, the effectiveness of the proposed scheme is validated through experimental results.