The invention relates to a load stimulation device of a servo system and a performance testing device of the servo system and belongs to the field of detection technology and automation technology. The devices integrate three modes of inertia loading, servo motor loading and spring loading, can realize independent loading of three loading modes, two combination loading or simultaneous loading, and can stimulate various loading conditions, such as inertia load, elastic loading, unbalanced load, impact load, time change load and the combination of various loads. Acquired signals of a tested servo system, a torsion rotational speed sensor and an angle sensor are sent to a computer for processing so as to realize the static and dynamic performance test under different loading conditions.