Coordinating the interactive relationship between county-level urbanization and industriralization is a key issue in promoting urban-rural integration.Based on panel data from 1，835 county-level administrative units in China （excluding municipal districts） for the years 2000， 2010， and 2020， this study constructs the Industrialization-Urbanization Index （IU）.A quadratic polynomial regression model is employed to explore the nonlinear impact of IU on urban-rural income disparity and regional economic development， thereby identifying the optimal ratio range between county-level industrialization and urbanization rates. The findings reveal that the optimal range of the IU index is between 1.33 and 1.65， within which moderate industrialization leadership contributes to economic growth and narrows the urban-rural income gap. Moreover， a reasonable level of IU facilitates urban-rural integration， whereas excessive urbanization or severe lag in urbanization hinder this process. On one hand， delayed urbanization leads to excessive dispersion of rural enterprises， weakening the sharing effects among enterprises and hindering urban-rural economic synergy.On the other hand， excessive urban expansion can result in “pseudo-urbanization”， exacerbating rural capital depletion， widening the urban-rural income gap， and inhibiting regional economic growth. As of 2020， approximately one-third （567） of counties in China fell within the optimal IU range， while 46%（847） were below this range， and nearly a quarter （421） exceeded it.To promote urban-rural integration development at the county level， it is necessary to consider the actual progress of industrialization and urbanization in different regions， maintaining a moderately leading position in county de-agriculturalization to drive urbanization.