Dissimilatory nitrate reduction in marine sediment is one of the key nitrogen loss processes in the ocean. Dissimilatory nitrate reduction to ammonium (DNRA), unlike denitrification and anammox by which nitrate is reduced to N₂ and removed from the environment eventually, directly reduce nitrate to ammonium, could lead to eutrophication and water hypoxia afterwards. ¹⁵N labeled technique is the main method to investigate dissimilatory nitrate reduction processes in sediments. Accurate determination of ¹⁵${\rm {NH}}_4^+ $ in isotope enrichment samples is primarily required to evaluate the potential rate of DNRA. The commonly used method for the determination of ¹⁵${\rm {NH}}_4^+ $ at present is the hypobromite iodine oxidation-membrane inlet quadrupole mass spectrometer determination method. However, ³⁰N₂ as the final analyte of the method has two problems which lead to an analysis error: firstly, ³⁰N₂ determined can be significantly overestimated due to the O₂ interference; secondly, the low equilibrium rate of ³⁰N₂ in the detector could influence the precision of the method and low down the analysis speed. To solve the problems mentioned, a sodium hypobromite oxidation-sulfamic acid reduction method by which ¹⁵${\rm {NH}}_4^+ $ is transformed to ²⁹N₂ only and detected using membrane inlet quadrupole mass spectrometer afterwards (Redox-MIMS method) is reported in this article. The results indicate that the optical concentration of sulfamic acid is 80−100 mmol/L; the detection limit is 0.5 μmol/L and the precision (RSD) is 0.8%; the dynamic range of standard curve is 0−150 μmol/L. Comparing with the hypobromite iodine oxidation method, the Redox-MIMS method not only has the advantages of mild reaction conditions and easily obtained reagents, the memory effect of ³⁰N₂ in the detector can also be solved effectively for most of the produced ¹⁵N₂ is ²⁹N₂ which improves detection efficiency (2 min per sample) meanwhile. Determination results of rates of DNRA and the contribution of DNRA to all dissimilatory nitrate reduction processes in the Laizhou Bay sediments using both hypobromite iodine oxidation and Redox-MIMS methods shows no significant difference. These make the Redox-MIMS method an accurate and high-efficient method for determination of ¹⁵${\rm {NH}}_4^+ $ in isotope enrichment samples.