Unreasonable excitation, speed regulation, PSS and other control system parameters in electromechanical transient simulation of power system will lead to the weakening of control system stability and the reduction of simulation credibility. A practical diagnostic method for this problem is presented based on the dominant trajectory section damping ratio sensitivity. The dominant time section, the dominant generator and its corresponding control system are determined automatically by the output variable oscillation characteristics of each control system in the simulation. After the single-machine infinite (OMIB) equivalent is performed for the dominant time section and the dominant generator, the suggestion of parameter adjustment is given by the sensitivity analysis of damping ratio. The effectiveness of the proposed method is verified by several examples. This method is an engineering application of the idea of trajectory eigenvalue. In combination with the oscillation information of the control system provided by numerical integration and the damping ratio sensitivity analytical information provided by eigenvalue technology, unreasonable control system parameters can be quickly located. It is suitable for parameter diagnosis of complex multi-machine system.