The invention discloses a transformable display screen and a 3D display device thereof. The transformable display screen switches between a reflective type and a transmissive type. The reflective transformable display screen is composed of a polymer-dispersed liquid crystal layer, a transmissive directing diffuser screen, and a reflector, or alternatively composed of a polymer-dispersed liquid crystal layer and a reflective directing diffuser screen; and the transmissive transformable display screen is composed of a polymer-dispersed liquid crystal layer and a transmissive directing diffuser screen. The 3D display device based on the transformable display screen comprises a high-speed projector, a reflector system, and a transformable display screen, and is capable of switching between spatial volumetric 3D display and spatial panoramic parallax 3D display. With no electric field being applied, the polymer-dispersed liquid crystal layer is ivory-white and severs as a projection diffuser screen to realize volumetric 3D display; and under an electric field being applied, the polymer-dispersed liquid crystal layer is transparent, thereby realizing panoramic parallax 3D display. The two types of 3D display can both fulfill the requirement from an adequate number of viewers for 360 DEG naked-eye observation, and can be switchably realized with complementary advantages favoring observation and locating of 3D spatial objects.