Recent studies in PM_2.5 sources show that anthropogenic emissions are the main contributors to haze pollution. Due to their essential roles in establishing policies for improving air quality, socioeconomic drivers of PM_2.5 levels have attracted increasing attention. Unlike previous studies focusing on the annual PM_2.5 concentration (C_year), this paper focuses on the accumulation phase of PM_2.5 during the pollution episode (PMAE) in the Yangtze River Delta in China. This paper mainly explores the spatial variations of PMAE and its links to the socioeconomic factors using a geographical detector and simple linear regression. The results indicated that PM_2.5 was more likely to accumulate in more developed cities, such as Nanjing and Shanghai. Compared with C_year, PMAE was more sensitive to socioeconomic impacts. Among the twelve indicators chosen for this study, population density was an especially critical factor that could affect the accumulation of PM_2.5 dramatically and accounted for the regional difference. A 1% increase in population density could cause a 0.167% rise in the maximal increment and a 0.214% rise in the daily increase rate of PM_2.5. Additionally, industry, energy consumption, and vehicles were also significantly associated with PM_2.5 accumulation. These conclusions could serve to remediate the severe PM_2.5 pollution in China.