The invention provides an electric servo mechanism load simulation system and a simulation method thereof. The system comprises a pedestal, a tested servo mechanism and a force moment loading mechanism which are disposed on the pedestal, a rigidness simulation mechanism, a friction simulation mechanism, an inertia simulation mechanism, a torque sensor, an angle sensor, and a central processing unit, wherein the rigidness simulation mechanism, the friction simulation mechanism, the inertia simulation mechanism and the torque sensor are disposed between the tested servo mechanism and the force moment loading mechanism, and the angle sensor is used for measuring the position and speed of the force moment loading mechanism. The rigidness simulation mechanism is connected with a rotor of the tested servo mechanism, and is driven by the rotation of the tested servo mechanism to swing. The friction simulation mechanism, the inertia simulation mechanism and the force moment loading mechanism are located in the same main axis, and are respectively used for simulating a friction load, an inertia load and an elastic load. The central processing unit receives detection signals of the torque sensor and the angle sensor, and judges whether the force moment parameter indexes of rotation inertia, rigidness, rotation speed and simulation position meet the requirements of performance testing under the condition of different loads or not.