This paper proposes a method to monitor atmospheric HCHO and CHOCHO with high temporal resolution based on differential optical absorption spectroscopy (DOAS) in Shanghai urban area. Based on the characteristic absorbing structure of HCHO and CHOCHO, different fitting intervals were chosen for spectral analysis in order to avoid the absorption of interfering gases and reduce the residuals of spectral analysis. The resulting optical thickness of the target gas is used to obtain HCHO and CHOCHO concentrations, which were averaged at (4.0±1.6) and (3.4±1.2) μg·m-3 in October 2013, respectively. The averaged concentrations of HCHO in workdays were higher than those in holidays due to the impacts of its anthropogenic emission sources, while no obvious differences of averaged CHOCHO concentrations between workdays and holidays were observed. Diurnal patterns of HCHO and CHOCHO were alike. In the early morning, both the HCHO and CHOCHO concentrations peaked at 06:00—07:00, and then decreased rapidly to the minimum around 09:00. Afterwards, the concentrations increased continuously until sunset and kept in a relatively stable level in the evening. To explore the possible emission source and formation mechanism of atmospheric HCHO, four typical periods, i.e. steady-state stage at night, morning rush hours, photochemical reaction stage and evening rush hours, were classified for source apportionment.NO2 is regarded as the indicator for primary source of ambient HCHO.As the intermediate products of photochemical reactions, HCHO has a similar formation mechanism in common with CHOCHO. Therefore, it is reasonable to use CHOCHO as an indicator for secondary source of ambient HCHO. The linear regression analysis showed a good agreement between modeled and observed HCHO concentrations, the correlation coefficients R2 ranged from 0.60 to 0.81. Secondary sources of HCHO were estimated to contribute to one third of ambient HCHO concentrations in Shanghai urban area.