Earthworms, the most important macro-fauna in terrestrial ecosystem play an important role in organic matter decomposition and nutrient cycling. The influence of the earthworm (Eisenia foetida) on these processes was evaluated under a laboratory experiment in which wheat straw has been incorporated into soil under three different treatments (incubated with earthworms at zero (ZDT)，low (LDT, as earthworm density of arable land in Quzhou experimental station, Hebei Province) and high density (HDT, three times higher than LDT). Each treatment was regularly destructively sampled to determine decomposition rates of residue and changes of nitrogen. The results showed that decomposition rates were accelerated by the increasing of Eisenia foetida density. Compared to ZDT, earthworm significantly increased the decomposition rates (P<0.05) during the prophase of the experiment (days 0-14). During the decomposition process, the nitrogen mineralization rate was slowed down in the HDT and LDT treatments, and was almost similar in the anaphase of the decomposition process（HDT:24.2-14.0 kg·hm-2·a-1，LDT:20.3-10.7. kg·hm-2·a-1）, so in the long run, the increase of earthworm density at farmland level would not increase the nitrogen mineralization rate.