The present invention relates generally to the field of ion storage and analysis, in particular to a linear ion trap mass analyzer comprised by multiple columnar electrodes. High frequency voltages are applied on at least one of the columnar electrodes to form ion confining space, which mainly consists of two-dimensional quadrupole electric radial trapping field, and there is at least one through slot for ion ejection in at least one direction perpendicular to the axis of the ion trap, wherein an AC electric field superposition is applied to invoke dipole excitation. Opposite to the through slot, there is an elongated electrode for field adjusting between two columnar electrodes or inside the slit of one of the columnar electrodes mentioned above. The potential on the elongated electrode for field adjusting is set as the sum of a portion of the high frequency voltage which applied on one adjacent columnar electrode and a DC offset, which can be adjusted freely. Through adjusting the portion of the high frequency potential and DC potential on this electrode, one or more objectives, including field optimization inside the ion trap as well as ion motion characteristics of resonant ejection, can be realized.