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Although China's western cities have developed rapidly in recent years, their ecosystems remain fragile. The conflict between urban development and environmental protection must be resolved to achieve the goal of sustainability. This study examines Xi'an as a case study of a megacity in Western China, calculating the city development index (CDI) and per capita ecological footprint (ef) from 2005 to 2016 to comprehensively calculate Xi'an's eco-efficiency. The E-C elasticity coefficient, which refers to the ratio of the rate of change in CDI to the rate of change in ef , is proposed to characterize the degree to which urban development responds to changes in the ef. The CDI and ef both showed an upward trend from 2005 to 2016. Xi'an's environmental efficiency has grown rapidly, and resource efficiency is the main factor driving its fluctuations in eco-efficiency. The energy consumption and pollution emissions are highly correlated with the CDI, and they are the main factors affecting the development of Xi'an. The relationship between CDI and ef is consistent with the law of diminishing marginal utility, the level of urban development rises and then decreases with the increase in ef. • City development index is significantly related to ecological footprint. • Main factors affecting city development are energy consumption and pollution emission. • Resource efficiency is the main driver of eco-efficiency change in Xi'an. • Development of Xi'an rises and then falls with the increase in ecological footprint. • E-C elasticity coefficient was proposed to illustrate the process of city development. [ABSTRACT FROM AUTHOR]