Abstract: The influence of a streamer background with nested close magnetic structures on the properties of CME initiated from its bottom is investigated in a solar meridian plane. In the streamer there contain three small-scale close magnetic field structures with the magnetic direction of the middle one opposite to that of the solar global dipole. A CME triggering model made up of cocircular magnetic field lines is taken to emerge from the solar equator at the inner boundary, where the middle small-scale close magnetic field structure locates, of the computational domain. The magnetic field direction in the front half of the CME triggering model with a radius a = OARs （Rs is solar radius） is opposite to that of the magnetic field line at its emerging position and same to that of the streamer global dipole. It is found from numerical simulation that： the CME triggering model, with a ratio of plasma pressure at its center and that at its edge m ≥ 2, can trigger a CME and can not when m 〈 2. The error for this numerical conclusion is less than 1%.