In this paper, a new rational approximation method for fractional-order differentiator sα and integrator s-α ( 0 < α <1) is presented. The method is based on the joint optimal rational approximation definition in the given frequency range and the error. First, a definition of joint optimal rational approximation is proposed and the constructing steps of the fractional-order integrator are discussed. Then the approximation algorithm of fractional-order integrator based on the joint optimal rational approximation definition is given. The construction steps of fractional-order integrator are explained by an illustrative example. Finally, the performance comparison result among the proposed approximation model, the Oustaloup’s method and the best rational approximation method shows that the amplitude-frequency characteristic and phase-frequency characteristic of the joint optimal rational approximation method are better than the Oustaloup’s method and the best rational approximation method. And the proposed model has higher approximation precision.