The preventive control strategy can be solved quickly and effectively through the linear programming model based on active power sensitivity. However,the active power sensitivity obtained by the DC power flow method ignores the influence of voltage and network loss,which makes the control strategy checked by AC power flow difficult to meet the accuracy requirement. For this reason,an online decision-making method for overload preventive control of transmission equipment based on the control objects relaxation is proposed. Firstly,the key equipment is determined by overload security margin,and the active power sensitivity of adjusting measures to key equipment under different post-fault operating conditions is calculated. Secondly,the active power adjusting measures are clustered. Thirdly,a linear programming model for overload preventive control online decision-making is established. Finally,the active power limit of key equipment is relaxed according to the active power accuracy of preventive control,and the search direction of the optimal solution is determined by AC power flow calculation. The correctness and effectiveness of the proposed method are verified through the analysis of the actual power grid.