In this paper, the dynamic failure of the two-phase ejector in a modified auto-cascade freezer system was experimentally investigated in the pull-down process. The effects of the critical ejector geometries including the nozzle throat diameter, mixing section diameter and nozzle exit position on the ejector performance were discussed in detail. The entrainment ratio and pressure lift ratio were used to evaluate the ejector performance. The malfunction behaviors of the pressure rise and fluid entrainment at different ejector geometries were analyzed. The results showed that two typical malfunction modes, i.e., pressure lifting failure and refrigerant entrainment failure, occurred in the pull-down process when ejector geometry were improperly adopted. The operation reliability of the ejector was sensitive to the nozzle throat diameter. Low mass entrainment ability and poor mixing effect for the mixing fluids were the main reasons of pumping failure for the ejector at undersize and overlarge mixing section diameters, respectively. Pressure lifting failure was prone to occur at smaller NXP, and the pulsation of the pressure lift ratio became obvious at large NXP when the secondary fluid was at two-phase state. The variation of temperature at the secondary fluid inlet was an important symbol of detecting the reverse flow of the primary from the secondary fluid inlet. The ejector with throat diameter of 0.8 mm, mixing section diameter of 3 mm and NXP of 6 mm was proposed in consideration of the ejector operation reliability in the pull down process, and the time-average value of the pressure lift ratio could reach 2.197 at the entrainment ratio of 0.663.