The calamus type surface-flow constructed wetland (SFCW) is widely used for treating low-polluted water bodies, such as effluents from wastewater treatment plants. The calamus biomass is considered a potential carbon source for denitrification, integrating plant biomass into the carbon cycle in SFCWs. This study conducted experiments on the calamus biomass decomposition and evaluated its potential application for denitrification in an SFCW system. We showed that decomposition of the biomass could release a high ratio of total organic carbon (TOC) to total nitrogen (TN), reaching 26.9, 27.7, and 23.6 at biomass doses of 0.1, 0.5, and 1.0 g L⁻¹. A continuous-flow experiment on the SFCW system revealed that higher TN removal efficiency and load were obtained after the addition of biomass, approximately three times before the biomass addition. Microbial community analysis revealed an increase in heterotrophic denitrifying bacteria (i.e., Flavobacterium, Dechloromonas, and Bacillus) content throughout the period of biomass addition. This study demonstrated an excellent enhancement of the denitrification efficiency in the SFCW caused by the addition of calamus biomass, suggesting its potential application as an excellent plant carbon source for biological denitrification.