A systemic research were carried out on the problem that the lithium niobate (LN) electro-optic (EO) Q-switches have poor consistence and can not completely hold-off in pulse-off Q-switching mode. The pulse-off EO Q-switching performances of several LN Q-switches were investigated in a flash-lamp pumped Nd∶YAG laser. The possible causes for the light leakage were analyzed according to EO Q-switching theory. On these basis, the axial deviation and optical inhomogeneity were characterized by a X-ray diffraction goniometer and the conoscopic interference method. The EO inhomogeneity of LN crystals was first found, the direction of main induction axis deviated from the theoretical 45° and the induced birefringence was inconsistent throughout the clear aperture. Through contrast experiments, the EO inhomogeneity is verified to be the main reason to cause the light leakage and poor consistence.