The integration of large-scale wind farms incurs new oscillation stability issues in the power systems. At present, the wind farm is always replaced with a single unit for the oscillation stability analysis in most literature. However, it ignores the dynamic interaction inside the wind farm and may lead to unreliable analysis results. This paper decomposes a wind farm with M coupled permanent magnetic synchronous generators (PMSGs) into a diagonal matrix with M blocks representing equivalent PMSG subsystems through the matrix transformation. On this basis, an impedance-based reduced-order model for the wind farm is derived by the matrix perturbation theory, which fully considers the topology and the dynamic characteristics inside the wind farm. Moreover, the proposed equivalent method can significantly reduce the complexity of analytical modelling while ensuring its accuracy, which is suitable for real-time oscillation stability analysis of large-scale grid-connected wind farm. The simulations results show the effectiveness of the proposed method.