Abstract: Exploring the basic public service supply quality and its influencing factors in northwest counties is of great significance for promoting the equalization of the basic public service. Taking 194 counties in northwest China as examples, this article analyzed the dynamic evolution characteristics and difference of the basic public service supply quality in northwest counties and explored their influencing factors by the Dagum Gini Coefficient and the spatial Durbin model. Results show that: the basic public service supply quality in northwest counties has steadily increased, from 22.73 in 2005 to 45.28 in 2020, with the overall level of Shaanxi > Gansu > Ningxia > Qinghai, and the spatial distribution pattern has gradually evolved from "high in the east and west, low in the middle" to "high in the east and low in the west, high in the northwest and low in the southwest". The inter-regional difference was the main source of overall difference, with the largest difference between Shaanxi and Qinghai. At the direct effect level, industrial structure upgrading, fiscal self-sufficiency, urban-rural income ratio, urbanization level, government administrative capacity, and population density had significant promoting influences on the supply quality. At the indirect effect level, industrial structure upgrading, fiscal self-sufficiency, economic development level, and population density had positive spillover effects. Therefore, it is recommended to identify the focus of improving supply quality, to pay attention to the adequacy of the basic public service supply, to attach importance to the coordination of basic public services, and to deeply promote the equalization of basic public services.