The invention discloses an automatic control system stability diagnosis and parameter adjusting method for electromechanical transient simulation and belongs to the electric power system simulation technology field. According to the method, through statistics of the continuous step number surpassing the designated iteration number during each step iteration convergence, or through identifying weak damping oscillation or amplification oscillation in a control system output variable during simulation, whether automatic stability diagnosis of a control system starts is determined; according to the output variable iteration process information of the control system during each step iteration, or according to magnitude of mean square deviation of the output variable of the control system in a time interval from fault elimination to automatic stability diagnosis of the control system, the dominant instability control system and a generator thereof are determined, through characteristic root analysis, damping ratio sensitivity of a dominant instability control system parameter is calculated; the parameter is adjusted according to the damping ratio sensitivity information. The system is advantaged in that electromechanical transient simulation credibility of an electric power system can be improved, and safety stability analysis of the electric power system and controller design are facilitated.