Abstract: In order to investigate the temporal-spatial distributions of DON in the shallow shelf of the East China Sea (SSESC), two cruises were implemented in late spring to early summer (25 May to 5 June) and in fall (18 October to 29 October) 2014, respectively. At the same time, the microcosm experiments on DON biodegradation were conducted on the ship board synchronously to evaluate the bioavailability of DON in the two seasons. The investigation results showed that DON comprised the majority of TDN in SSECS. The average proportion of DON to TDN in late spring to early summer and in fall were 57.88%±21.42% and 66.09%±19.74%, respectively. The spatial distributions of DON were the consequence of joint contribution of the terrigenous input, the Kuroshio upwelling as well phytoplankton secretion and bacterial degradation. During late spring to early summer, DON concentration decreased from inshore to offshore, and the regions with high value of DON were consistent with the direction of the Yangtze River diluted water (YZRDW). While in fall, the regions with high value of DON were located in the northeast and southeast of the SSESC. On a whole, the average DON concentration in the north of SSESC was higher than that in the south, due to the influence of the YZRDW. From the surface layer to the bottom of the water column, the supply of DON by the phytoplankton photosynthesis decreases while degradation of DON by bacteria increased, resulting that the concentration of DON and its proportion to total dissolved nitrogen (TDN) decreased gradually. The temporal-spatial distribution of bioavailability of DON also varied significantly. The labile DON (L-DON)in the surface layer showed a decreasing pattern from inshore to offshore. The proportion of L-DON to DON in the region influenced obviously by terrigenous input accounted for 28.3%in late spring to early summer and 18.9% in fall, which was significantly higher than that in the region influenced weakly by terrigenous input.