This paper aims to develop a small-signal stability analysis method for large-scale power electronics-based power systems. For this purpose, the nodal admittance matrix (NAM)-based approach is recognized as the most precise technique. However, the original implementation of NAM method is tailored for the entire system, thereby correlating the matrix dimensions with the number of converters present in the system. Consequently, it becomes impractical to directly apply the original NAM method to a large-scale system. To address this challenge, this paper introduces a novel system-partitioning-based NAM approach. In this method, the large-scale system is decomposed into several subsystems first, followed by analysis at the interconnection level. The general concept, the detailed mathematical derivation, and the applications of the proposed method to a 6-converter system and a modified 140-bus NPCC system are presented. It has been validated that the proposed approach can significantly reduce computational burden while simultaneously preserving the accuracy for large-scale PE-rich power systems.