The invention relates to a motor load aggregation method for an electric system and belongs to the field of load modeling for the electric system. The method includes the steps that firstly, according to main motor parameters affecting the stability of the electric system, two groups of good system stability and poor system stability are obtained through division; then, according to actual operation conditions of all motors, electromagnetic and mechanical characteristics of motor rotors are comprehensively considered, the initial slip frequency for an aggregated motor is obtained through solving, and then electric parameters of the aggregated motor are obtained through solving and include stator reactance, rotor reactance, excitation reactance, rotor resistance and an inertia tie constant; finally, the method is verified through a calculation example model. The result shows that the method is high in aggregation precision and small in calculation amount, original motor groups are simulated through two equivalent motors adopted for aggregation, errors are smaller than the errors caused by the adoption of a single equivalent motor, and the method is suitable for transient stability analysis.