Abstract: Studying the spatio-temporal evolution of county-level economic development and its influencing factors is significant for narrowing regional disparities and promoting coordinated development of county economies. Taking the counties within the Chengdu-Chongqing Economic Circle from 2012 to 2023 as the study area, this research integrates comprehensive economic indicators with nighttime light data to measure county-level economic development. It employs spatial pattern analysis methods to investigate changes in the economic spatio-temporal pattern and utilizes various econometric models to analyze regional differences and spatial spillover effects of the factors influencing economic development. The findings reveal that: (1) The Chengdu-Chongqing Economic Circle exhibits "high-high clustering" centered on the central urban areas of Chongqing and Chengdu, with an overall increasing agglomeration effect. (2) The economic circle displays an "east-west" pattern, with its center of gravity shifting northwestward, indicating faster economic growth in the northwestern region. (3) Population size is the most critical factor, followed by market dynamics and industrial structure as secondary influences, while education level, government regulation, and financial development are important factors, with their impacts showing heterogeneity based on the level of economic development and regional constraints. (4) The spatial spillover effects of these influencing factors are greater than their direct effects, meaning the factors have a more substantial impact on the economic development of neighboring regions than on their own. Recommendations are proposed: First, strengthen the leading role of the dual core cities. Enhance inter-regional transportation and network infrastructure to facilitate factor mobility. Second, promote integrated development of the northern and southern wings. Advance the construction of the Wan-Da-Kai Sichuan-Chongqing Coordinated Development Demonstration Zone and establish a pilot urban area for southern central development. Third, adopt tailored measures. Chongqing should strengthen its market system and focus on developing advanced manufacturing. Sichuan should enhance government regulatory capacity, foster regional market coordination, and develop high-tech industries. Fourth, encourage coordinated regional development. Promote market circulation, technological exchange, and industrial cooperation among cities within the region to leverage their strong spatial spillover effects.