Effects of dissolved inorganic nitrogen (NO₃-^{-N and NH₄+-N) addition on dissolved inorganic carbon (DIC) system of Saccharina japonica culture water were studied. The results indicated that the inorganic carbon system was correlated with the concentrations and forms of DIN. The concentrations of DIC, HCO₃- and pCO2 decreased with the increasing nitrogen concentration in the range of 4.73-52.78 µmol/L (NO₃--N) and 2.5-34.66 µmol/L (NH4 -N), and they were most remarkably affected by the NO₃--3 and NH4 -3 treatments, corresponding with the lowest values of 2054 and 2112 µmol/L, 1776 and 1869 µmol/L, 86 and 114 µatm, respectively. However, when the concentration of nitrogen was between 52.78-427.29 µmol/L (NO₃--N) and 34.66-268.33 µmol/L (NH4 -N), the falling trends of DIC, HCO₃- and pCO2 were weakened with the increasing nitrogen, but the concentrations of DIC, HCO₃- and pCO2 were also lower than the control. Influence of NO3--N addition on the inorganic carbon system of seawater was larger than NH4 -N addition, and the ability of carbon fixation in NO₃--N treatment was significantly higher than in NH4 -N treatment. The highest capacity of photosynthetic carbon fixation by S. japonica appeared in 52.78 µmol/L (NO₃--N) and 34.66 µmol/L (NH4 -N). The DIC assimilation ability by S. japonica would decrease when NO₃--N and NH4 -N deviated from the optimum values.}