The invention discloses a method and system for determining the stability of a system transient power angle. The method comprises steps of: acquiring the speed information of a generator at the current sampling time and the previous sampling time, and separately calculating the speed information deviation data of the generator at the current sampling time and the previous sampling time by using the obtained speed information at the current sampling time and the previous sampling time and the rated speed information of the generator; calculating the derivatives of the speed information deviation data of the generator at the current sampling time and the previous sampling time; determining whether the speed information deviation data of the generator at the current sampling time and the previous sampling time and the derivatives of the speed information deviation data satisfy a stability determination condition; if so, determining that the system transient power angle is unstable at thecurrent time; otherwise, determining that the system transient power angle is stable at the current time. The method can determine the transient power angle stability of the system only by the speed information (rotation speed or machine-end frequency) of the generator, and is simple and feasible in engineering and easy to implement.