The utility model discloses an external type PSS device. There is the field regulator of some at present, dispose PSS or still adopt PSS the operation of 1A model, the transformation fixed number of years is unable whole to be reformed transform owing to not arriving. The utility model discloses a characterized in that, the DSP plug -in components pass through bus board and AC input plug -in components, DA output plug -in components and intelligent IO plug -in components link to each other, it gathers the signal to obtain the generator stator voltage current mode volume of planning, the switching value opens the signal, it calculates to carry out the PSS model, logic control, and pass through the analog quantity bus with the analog quantity numerical result and carry to the analog quantity port of DA output plug -in components and the optical fiber communication port of DSP plug -in components, leave port output with the quantity of state signal through IO bus transport to intelligent IO plug -in components. The utility model discloses dispose independent PSS device outside the regulator, reduce the requirement to PSS model in the field regulator, relative field regulator cost is lower, to the unsatisfied excitation system that has put into operation who requires of PSS, can reduce the equipment investment that excitation system PSS reformed transform.