Currently, the effects of biochar on the accumulation of soil organic carbon (SOC), especially microbial necromass carbon (MNC), during its application in coastal wetland soil improvement, are still unclear. To address this issue, a 100-day indoor soil incubation experiment was carried out with 6 kinds of biochars prepared under different conditions using municipal sewage sludge and Entermorpha Prolifera as raw materials. The results showed that compared with the CK, the high temperature-prepared (700℃) sewage sludge biochar, especially for the sodium alginate modified sewage sludge-Entermorpha prolifera co-pyrolytic biochar (SA-SEB7), significantly increased SOC content, but obviously decreased MNC content. By using phospholipid fatty acid (PLFA) method to analyze the microbial biomass and community structures of different treatments, it was found that the total content of PLFAs changed insignificantly under the treatments added with high temperature-prepared sewage sludge biochars, while significantly increased under the treatments added with low temperature-prepared sewage sludge biochars, especially for the sodium alginate modified sewage sludge-Entermorpha prolifera co-pyrolytic biochar (SA-SEB3). This outcome implied that soil MNC generation may be marginally affected by the high temperature-prepared sewage sludge biochars, but greatly promoted by the low temperature-prepared sewage sludge biochars. Furthermore, under the treatments added with high temperature-prepared sewage sludge biochars, the relative abundance of K-strategy bacteria such as Actinomycetes increased, while that of r-strategy bacteria such as gram-negative bacteria decreased. This shift in microbial community may cause K-strategy bacteria to reuse MNC produced by r-strategy bacteria as a nitrogen source required for their own growth, and then reduced the accumulation of MNC. In summary, SA-SEB7 has greater potential to increase SOC content in coastal wetland soil, and its main contribution is from the stable organic carbon input of biochar itself, rather than by promoting the accumulation of MNC.