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As a characteristic biogenic volatile organic compound, isoprene is directly emitted by plants and participates in the atmospheric chemical process promptly. In virtue of differential optical absorption spectroscopy (DOAS) technique, isoprene was measured from spring to autumn of 2018 in urban area of Shanghai, China. The concentration of isoprene reached its peak in July with monthly average exceeded 0.3 ppbv. Besides, a distinct diurnal variation was observed and peaked around 13:00, up to 0.6 ppbv. The observed isoprene was found closely related to meteorological factors, and showed a good correlation with emission active factor γ calculated by temperature and radiation (R2 = 0.87), indicating that the urban isoprene emission is mainly emitted from biogenic sources and regulated by temperature and radiation. According to the proportional relationship between isoprene and its major intermediate oxidation products, it suggested that vast majority of daytime methyl vinyl ketone (MVK) and methacrolein (MACR) were derived from the oxidation of isoprene. To further understand the oxidation processes of isoprene, six cases covering different atmospheric conditions were selected to simulate atmospheric radicals relied on the box model. The daily peak concentration of simulated ·OH and NO3 ranged from 5.98 × 106 to 1.19 × 107 molecule cm−3 and 2.35 × 108 to 1.49 × 109 molecule cm−3, presenting a medium-to-high level of atmospheric oxidation capacity. And oxidants of ·OH, O3 and NO3 shared 66–95%, 1–4% and 3–30% of isoprene oxidation, respectively. The comparison of estimated initial and measured isoprene concentration implied that at least half of the isoprene was oxidized between emission and measurement. The short daytime atmospheric lifetime (0.4–1.1 h) also suggested the high reactivity of isoprene. This study indicates that the urban ambient isoprene is fast oxidized and fully involved in the atmospheric oxidation processes, presenting some subtle differences under different air pollution conditions.