With the rapid development of urban rail transit, the subway, as an important public transportation, provides an efficient largescale travel option. Previous studies generally believe that the built environment around subway stations affects the passenger flow, but the size of PCA (Pedestrian Catchment Area) has not yet been unified. The purpose of this study was to verify whether the size of PCA affects the outcome value of the correlation study between metro ridership and the built environment. Taking 25 stations of Nanning Metro Line 1 as an example, five indicators were selected as independent variables, including residential population, jobs and commercial facilities, and the actual passenger volume of the stations was used as the dependent variable. The OLS regression model was used to compare the results of model fitting and influencing factor analysis under different PCA radius variables. There were differences in the research results of the relationship between passenger flow and built environment in the data collection of PCA in subway stations within different catchment areas. For Nanning, the radius of the PCA is 600 m, and the fitting of the correlation model between subway passenger flow and built environment is better than that of 300 m and 900 m. The scope of PCA will lead to inconsistency in the results of the study on the association between subway passenger flow and the built environment. In the future, it is necessary to conduct model analysis for different PCA radii and determine the appropriate PCA scale according to the fitting effect to improve the accuracy of the research.