Interethnic marriage is a relational phenomenon. It not only reflects individual marital choices but also forms a network of social ties among ethnic groups. Existing studies on interethnic marriage in China have mainly relied on intermarriage rates or regression models of marital choice. These approaches describe the level and determinants of interethnic marriage, but they do not fully reveal how the structure changes over time. An increase in the total volume of interethnic marriage may result either from more ethnic-group pairs becoming connected or from the accumulation of marriages within ties that have already existed. Distinguishing these two processes is essential for understanding the structural change of interethnic relations in China.

This study applies an intensity-connectivity framework to analyze changes in interethnic marriage. Drawing on Das Gupta's standardized decomposition method, it decomposes the total volume of intermarriage into two dimensions: intensity, defined as the average number of interethnic marriages per connected ethnic-group pair, and connectivity, defined as the proportion of connected ethnic-group pairs among all possible ethnic-group pairs. Based on interethnic marriage matrices for 56 ethnic groups from the 2000, 2010, and 2020 national population censuses, the study examines both aggregate network change and ethnic-group-level heterogeneity. Because the census data are stock rather than flow data, both measures describe accumulated relational structures rather than newly formed marriages.

The results show that China's interethnic marriage network underwent a structural change from breadth expansion to depth accumulation between 2000 and 2020. The total volume of interethnic marriage increased from 8.04 million in 2000 to 9.64 million in 2020, but the structural sources of this growth differed across periods. From 2000 to 2010, connectivity accounted for 93.8% of the increase, indicating that growth was driven primarily by the formation of marriage ties among previously unconnected ethnic groups. From 2010 to 2020, intensity contributed 120.9% of the increase, while the contribution of connectivity became negative. In this period, growth was driven by the accumulation of marriages within existing ties, whereas the decline in connectivity may reflect the disappearance of some weak or low-frequency ties. Accordingly, network connectivity followed an inverted U-shaped trajectory, rising from 2000 to 2010 and declining from 2010 to 2020.

At the ethnic-group level, changes in interethnic marriage were heterogeneous. The study identifies four types of structural change: depth accumulation, breadth expansion, simultaneous depth and breadth expansion, and marriage contraction. Most ethnic groups fall into the depth accumulation type, indicating that their growth in interethnic marriage mainly came from the deepening of existing ties. A small number of groups showed continued expansion in their relational scope, while others experienced both expansion and deepening. For a few groups with small populations and sparse ties, the loss of a few existing marriages may lead to contraction.

These findings show that changes in interethnic marriage cannot be adequately understood through intermarriage rates alone. The intensity-connectivity framework reveals the reorganization of the scope and accumulated depth of interethnic marriage ties, while the typology suggests that differing network positions should be fully considered when promoting interaction, communication, and integration among all ethnic groups in China.