The formation mechanism and evolution process of secondary organic aerosol (SOA) remain poorly understood due to measurement uncertainties and chemical complexities. Here, we present the characterization of non-refractory fine particles (NR-PM2.5) analyzed by means of a time-of-flight ACSM (ToF-ACSM) at a rural site in the North China Plain during winter. Our results show that air quality in rural areas was heavily polluted by primary organic aerosols (POA), which accounted for 83% of total organic aerosol (OA). The oxygenated organic aerosols (OOA) were mainly generated from local emissions. As pieces of evidence, firstly, less-oxidized OOA (LO-OOA) had good correlations with primary species like biomass burning OA (BBOA) (R2 = 0.54), EC (R2 = 0.46), and chloride (R2 = 0.46), implying LO-OOA was formed with the POA emission processes; secondly, there was a significant increase in OOA concentration after sunrise, indicating a lot of local generation during the daytime; thirdly, the potential source region of OOA was restricted in that of POA, implying that OOAs were converted from where primary emission exists. The formation processes of LO-OOA and more-oxidized OOA (MO-OOA) are different. Aqueous-phase chemistry had a dominant effect on the formation of LO-OOA due to the better correlation between aerosol liquid water content (ALWC) with LO-OOA (R2 = 0.54) than MO-OOA (R2 = 0.15). In comparison, both aqueous-phase and photochemical processes acted on the MO-OOA formation, when odd oxygen (Ox = O3+NO2) was low (Ox < 35 ppb), MO-OOA concentration increased with the increment of ALWC. However, at high Ox level (Ox > 35 ppb), photochemical oxidation played an essential role in MO-OOA formation.