To study the stability and active power of the synchronous generator, this paper deduces the electromotive force balance equation of its armature reaction by use of its time & space vector diagram. Based on this, through the result of the operation characteristics experiment of DDSZ–1 salient pole synchronous generator connected to the three-phase symmetrical load, it analyzes the influence of its rotor magnetic circuit structure on operation stability, and draws the conclusion that the smaller the rotor air-gap is the poorer its stability of parallel operation, vice versa. Finally, with synchronous generator power angle of time and space vector diagram, its electromagnetic power calculation formula and the influence of the rotor air-gap on active power are deduced, i.e. increasing the rotor air-gap to each space direction proportionally can increase its active power.