Bentonite and wormcast were used as mulching material, and indoor simulation experiments were carried out to study the effect of the superstratum of pile on preserving nitrogen during municipal sludge composting. The results showed that wormcast combined with bentonite could play a good role in preserving nitrogen during municipal sludge composting. The effect of covering the pile with wormcast firstly and bentonite secondly was better than that of covering the pile with bentonite firstly and wormcast secondly. Compared to control, the accumulative amount of ammonia emission of the treatment of covering the pile with wormcast firstly and with bentonite secondly decreased by 79.58%.