By means of the diffraction loss sensitivity of the diaphragm, the anti-alignment capability of ring laser gyro (RLG) is analyzed numerically. Deriving from the condition of mode suppression of RLG, the lower limit of mode suppression ratio (MSR), which is defined as the ratio of the loss of a higher order transverse mode to that of the primary transverse mode, is calculated to obtain the optimal diaphragm diffraction loss sensitivity. The dependence of the lower limit of MSR (LLMSR) for a RLG on its gain level as well as its total mirror loss is investigated both numerically and experimentally, with the results showing that the higher the gain level, the higher the LLMSR, while the higher the total mirror loss, the lower the LLMSR. These results will be of great significance for the MSR optimizing control during the resonator alignment of RLG.