Abstract: Defines the factional order calculus and clarifies its basic characteristics because the fractional order systems have been used much wider than before in application. Generally the characteristic equation of a fractional order system is not a real polynomial but a pseudo-polynomial function of complex variable with fractional exponent, which can be approximated and converted into a high-order integral exponent system so as to study it by existing control approaches. A method is thus proposed to analyze fractional order linear systems on the basis of the definition of fractional calculus, and its effectiveness is verified via a simulation example.