Abstract: To solve the problem that the MSL system can not efficiently remove TN from polluted river water because of inadequate denitrification therein, the system was modified by using solid carbon from non-water-soluble, biodegradable PBS and optimizing its spatial structure. To determine nitrogen-removal effect of the modified MSL system, an experiment was carried out to compare the modified one with an old one using sawdust carbon, showing that granular PBS carbon is much better in denitrification effect, thus raising the TN removal rate up to 64.3%, 20.6% peretage point higher than the one using sawdust carbon, and the optimization of the spatial structure of the system uplifted TN removel efficiency from 43.7% to 71.7%, Moreover, the system worked more smoothly and free of clogging. However, the modification did not have any obvious effect on removal of organic matter|, phosphorous and ammonia nitrogen.