This study numerically investigated the effects of a deflector structure installed behind a disk-type cavitator on the development and hydrodynamic stability of a ventilated supercavitating flow. A multiphase volume-of-fluid model coupled with a large eddy simulation framework was employed. Numerical simulations were performed on test models with and without a deflector structure under conditions of identical Froude numbers and angles of attack of the cavitator. The results showed that the presence of the deflector structure accelerated supercavity development, thereby increasing the maximum cavity length by approximately 25%. Without a deflector structure, the supercavity exhibited strong unsteadiness and large-scale shedding, whereas the presence of the deflector structure stabilized the air–water interface and suppressed large-scale shedding by breaking it down into smaller vortical structures. The total drag acting on the model with the deflector structure was slightly lower than that acting on the model without a deflector structure, with a maximum drag reduction of approximately 2%. Under a nonzero angle of attack of the cavitator, the supercavity became increasingly asymmetric; however, the cavity length for the model with the deflector structure remained consistently greater. This study provides a systematic investigation of the role of deflector structures in supercavitating flows, offering new numerical insights into the underlying physical mechanisms and practical guidance for the hydrodynamic design of high-speed underwater vehicles.