Based on a simplified predator-prey model, the influence of noise on the ecosystem has been studied. The results show the following facts. (i) For all parameter values, the existence of noise maintains the oscillatory state of the ecosystem, and enough strong noise can destroy the ecosystem, which means the annihilation of the species. (ii) Comparing to oscillation with small amplitude, while the ecosystem oscillates explosively with large amplitude, it is more likely to lose balance. In addition, the small-amplitude oscillation takes on higher level of regularity. All the numerical results are reasonable comparing to the general knowledge about ecosystem.