Current modeling studies have underestimated secondary organic aerosol (SOA) levels in China to a larger degree than over Europe and the United States. In this study, we investigated the SOA formation pathways in urban Beijing for the period of November 7–8, 2014, using an observation-constrained box model in which the multigenerational oxidation processes of volatile organic compounds (VOCs) and intermediate VOCs (IVOCs) and the chemical aging of semi-volatile primary organic aerosols (POAs) were taken into account. The results demonstrated that the SOA formation rate was 30.3 µg m–3 day–1 in Beijing during the 2-day study period. The contributions of VOCs, IVOCs, and POAs to the SOA levels were 14%, 82%, and 4%, respectively. IVOC contributions were on a scale similar to the magnitude of underestimation in a previous study. The uncertainty analysis showed that SOA levels during the study period were 55.4–102.4 µg m–3 (the 25th and 75th percentiles of the sensitivity simulations). The contribution of IVOCs to the SOA formation was dominant compared with that of VOCs and POAs. A more precise IVOC oxidation mechanism can thus improve the performance of the SOA model in China.