Abstract: Small and medium-sized cities belong to the end of the urbanization system. With the slowdown and even reduction of country's total population growth, many county-level small and medium-sized cities will enter the era of contraction. The identification and analysis of county-level urban contraction phenomenon is conducive to reveal the urban contraction phenomenon in more detail and understand the development trend and influencing factors behind it. Based on the data of the fifth, sixth and seventh census and the urban development index data in the corresponding years, this paper identifies the contraction of county-level cities in Liaoning Province, and quantitatively analyzes the background factors affecting the contraction of county-level cities through grasping the contraction in different time spans. The research shows that the proportion of county-level cities in Liaoning Province shrinking from 2000 to 2010 is small, but there is an overall contraction of county-level cities and a large contraction of non-central urban areas from 2010 to 2020, including the process of urbanization, population age structure, rural development level regional economic strength and personal pursuit of matching are the main factors affecting the contraction of county-level cities in Liaoning Province.