The invention discloses an orthogonal excitation synchronous motor and a method for establishing a speeder reducer, a gearbox and a synchronous motor testing system by using the same. The technical effect of the invention is as follows: vector change of relative physical positions between the magnetic field direction of a synchronous motor rotor and the synchronous motor rotor is realized by the novel orthogonal excitation synchronous motor. According to the test scheme, mutual feedback loading of synchronous motor set armature can be adopted, and an external energy compensating device (mechanical or electric drive energy compensation) just needs to provide initial starting energy before loading for the system and compensates energy loss of the system in the operation process, therefore a full-power electric speed adjusting device or a mechanical adjusting device is not needed, the total investment of the system is greatly reduced, and the method is simple and convenient to control. When the orthogonal excitation synchronous motor operates, the power angle of the synchronous motor can be adjusted by adjusting the excitation to realize continuous adjustment of loading power; because the closed-loop control is carried out by an external feedback device, the loading control precision can be further ensured effectively.