Abstract: The changes of ¹³C content in ¹³C pulse-labeled Spartina alterniflora plants and root soil organic carbon were analyzed under different levels of nitrogen (N)and phosphorus (P). The effects of N and P addition levels on the distribution and fixation of photosynthetic carbon in S. alterniflora-soil system were studied. The results show that the abundance of ¹³C in each treatment group was significantly increased after four pulse marks. Except for the treatment of NP1, the abundance of ¹³C showed a decreasing trend in the order of stem, leaf, root, rhizosphere soil and soil, which was consistent with the control group (CK). The total amount of ¹³C fixed in each treatment group show a trend of continuous increase, but it was lower than that of the control group. The average fixed ¹³C amount of NP treatment was higher than that of N or P treatment alone. During the development of plants, the proportion of photosynthetic carbon in the aboveground gradually decreased while the proportion of underground gradually increased, and the accumulation of organic carbon ¹³C in soil was enhanced. With the development of plants, the differences in the distribution of ¹³C in each part of S. alterniflora-soil system became similar and tend to be consistent. In the N-added group, the allocation ratio of carbon in rhizosphere soil and soil increased with the increase of N addition level, indicating that N application could promote the transfer of photosynthetic carbon to soil. In the P-added group, the highest underground allocation ratio of photosynthetic carbon was observed in the medium level of P addition treatment (P2), which was conducive to the underground transfer of photosynthetic carbon. In the NP-added group, the aboveground distribution ratio was the highest at medium NP level (NP2), and the distribution ratio of rhizosphere soil and soil increased with the increase of NP addition. The addition levels of N and P and types of nutrients can change the distribution of photosynthetic carbon in S. alterniflora-soil system, indicating that eutrophication had significant ecological effects on the carbon cycle in salt marsh ecosystem.