Abstract: Dissolved oxygen (DO) is a key indicator for water environmental quality and water ecological health. Low level of DO poses a serious problem for improving water ecological environment quality in tidal reach. In this study, the tidal reach of the Mulan River was selected as the research area. The sediment oxygen demand (SOD) in different regions was studied under different temperature conditions. The distribution of oxygen consuming substances such as total organic carbon (TOC), total nitrogen (TN), Fe, Mn, ammonia nitrogen (NH₄⁺-N) and sulfur (S) in the sediments of different regions were analyzed, which clarified the influence of oxygen consumption substances on SOD level in sediments. The results showed that: (1) The SOD in the tidal reach of Mulan River was 3.06-16.64 mg/(m²·h) in spring and autumn, and 14.02-52.44 mg/(m²·h) in summer. The significant influence of temperature on the SOD was confirmed in this study. In addition, the results of summer revealed the grim situation of oxygen consumption in sediment. (2) The upstream river was affected by chemical oxygen depletion with high SOD in the low temperature season, while the estuary was affected by biological oxygen depletion with significantly high SOD in the high temperature season than upstream river areas. The SOD in the left and right banks of the Mulan River was higher than that in the middle. Moreover, the SOD in the mudflat aquaculture area was higher in summer. (3) The correlation analysis showed that the SOD had no significant correlation with TOC, TN or Fe in the sediments, the SOD in spring and autumn had significant positive correlation with Mn, and the SOD in summer had extremely significant positive correlation with NH₄⁺-N and S in the sediments. Besides, there was a significant negative correlation between DO concentration and SOD in summer. This study indicated that the temporal and spatial variability of SOD in the tide reach of Mulan River was obvious, and the sediment oxygen consumption rate was significantly higher in summer, which had a potential risk for low DO level in the overlying water.