Abstract: The AC grid strength of power systems integrated by renewable power generations via power electronic devices should meet the requirement due to the risk of system oscillation. However, due to the strong interaction among the power electronic devices, it is difficult to evaluate the AC grid strength. This paper explored the evaluation of grid strength from small-signal stability perspective. Firstly, we constructed the linearized model of multiple power electronic devices infeed systems（MPEIS） and derived the characteristic equation for the MPEIS. Then, based on eigenvalue perturbation theory, it is proved that the dominant characteristic locus of an inhomogeneous MPEIS can be equivalently represented by that of a homogeneous MPEIS which has the similar AC grid structure as the inhomogeneous MIPES. Finally, we extended the application of generalized short-circuit ratio（g SCR） from homogeneous MIPES to inhomogeneous MIPES, as well as two equivalent definitions and the calculation method of critical generalized short-circuit ratio（CgSCR）. The theoretical analysis results showed that the definition of gSCR of a homogenous MIPES is still valid for an inhomogeneous MIPES, while their critical values are different. The case studies illustrated the effectiveness of the proposed methods.