Abstract: This study investigated the concentration distribution, photoproduction rate, dark reaction rate, microbial consumption rate, and sea-to-air flux of carbon monoxide (CO) in the East China Sea, based on a field survey conducted in the spring of 2023. Results revealed that atmospheric CO mixing ratios ranged from 137 to 364 ppbv, with a mean of (205 ± 58) ppbv. In the surface seawater, CO concentrations varied from 0.18 to 3.02 nmol/L, with a mean of (1.4 ± 0.7) nmol/L. The vertical distribution of CO displayed higher concentrations in the surface layer, decreasing with depth, and exhibited noticeable daily variations across different water layers. In particular, the surface layer seawater showed CO supersaturation at all sampling stations, indicating the role of the East China Sea as a net source of atmospheric CO. The findings underscore the significance of coastal emissions when assessing global oceanic CO emissions. Moreover, the full-spectrum solar radiation photoproduction rate of CO in the surface seawater of the East China Sea during spring was calculated to be (67.67 ± 4.38) nmol/(L·d), the dark reaction rate as (3.70 ± 0.45) nmol/(L·d), and the microbial depletion rate constant as (2.34 ± 1.04) /d. Notably, the photoproduction of CO in the surface seawater during spring accounted for (196.96 ± 12.75) Gg·(CO-C), while dark reaction production and microbial consumption contributed (19.62 ± 2.04) Gg·(CO-C) and (23.98 ± 0.19) Gg·(CO-C), respectively. Combining these results with sea-to-air diffusive fluxes, it was estimated that the East China Sea released approximately (1.57 ± 1.67) Gg·(CO-C) of CO into the atmosphere during spring.