Carbon source is one of the limiting factors in denitrification process of low C/N ratio wastewater, while the addition of solid carbon source can enhance the microbial denitrification. In order to screen out suitable external carbon sources, this study selected cheap agricultural waste (straw and sawdust) and aquatic plants (Myriophyllum aquaticum and Pontederia cordata) as solid carbon sources, analyzed the carbon release characteristics of different solid carbon sources, and compared their nitrogen removal efficiency in the denitrification process. Results showed that the carbon release processes of four study materials all complied with the second-order kinetic equation, and their carbon release capacity was as follows: straw (25.64 mg/(g·L)) >P. cordata (23.64 mg/(g·L)) > sawdust (22.37 mg/(g·L)) > M. aquaticum (20.45 mg/(g·L)). Among the four materials, the release rate of M. aquaticum was the fastest, and the release time of its COD was only 3.56 h when its COD reached half the saturated concentration. The denitrification efficiency of the four materials as external solid carbon sources could be significantly improved, and the nitrate nitrogen removal efficiency of the four materials all reached more than 80%. Due to the large accumulation of ammonia nitrogen in the later stage of the experiment, P. cordata as solid carbon source will cause secondary water pollution. Therefore, straw, sawdust and M. aquaticum are suitable for the utilization as additional carbon source materials.