<p>This study empirically explores the impact of postgraduate education expansion on the income gap using panel data from 30 Chinese provinces within 2003–2022. To this end, it employs methods such as benchmark regression, threshold modeling, instrumental variable approaches, and heterogeneity tests. The results show that the expansion of postgraduate education scale significantly widens the income gap and exhibits a threshold effect associated with levels of economic development. After crossing this threshold, the income gap–widening effect of postgraduate education expansion gradually weakens and loses statistical significance. Economic development can inhibit the effect of postgraduate education expansion on enlarging the income gap. Regional heterogeneity analysis indicates that postgraduate education expansion in regions with low levels of economic development increases income inequality more prominently. Educational hierarchical heterogeneity analysis reveals that the negative impact of doctoral education expansion on the income gap is stronger than that of master’s education expansion. Finally, based on the empirical results and analysis, targeted policy recommendations are proposed to guide the development of postgraduate education and narrow the income gap.</p>