We report the differences between δ15N of organic nitrogen bound in foraminifera tests (foraminifera‐bound δ15N, or FB‐δ15N), which is considered an effective recorder of thermocline nitrate δ15N in the oligotrophic areas, and bulk sedimentary δ15N (δ15Nbulk) of core MD12‐3433, located in the northern South China Sea (SCS), to explore possible causes of the offset between these two proxies in the SCS. The offset can be best explained by the influence of the terrigenous input on δ15Nbulk, as evidenced by its general correlation with sea level changes, C:N ratios, and inorganic nitrogen contribution to total nitrogen. Moreover, our new record is compared to six previously published δ15Nbulk records of the SCS based on their geographical proximity to the mainland or large rivers, which all show being influenced by terrigenous input. The effect of changes in vertical mixing is evaluated and believed to play a minor role on the general pattern of FB‐δ15N, while N2 fixation is the most important impact on FB‐δ15N records in the SCS as previously studies revealed. A deeper mixed layer during the last glacial period could result in nitrate with higher δ15N from the subsurface being mixed into the euphotic zone. FB‐δ15N of Orbulina universa have higher values than Globigerinoides ruber and Globigerinoides sacculifer, with a maximum difference in the last glacial maximum, which could be accounted for by its deeper habitat where it is sensitive to changes in the contributions of vertically mixed subsurface nitrate relative to nitrate from N2 fixation.