A shield for a helmet is provided and the shield is served as a projection screen for a head-up display device. The shield includes an object lens, and an ocular lens. The ocular lens is positioned above the object lens. The object lens reflects an image displayed on the head-up display device at a front-bottom of the helmet, the image being reflected by the object lens and then projected to the ocular lens, after that, the image is reflected by the ocular lens to rear of the ocular lens. The aforementioned arrangement reduces electronic components attached to the helmet and further provides an embodiment in regard to the head-up display device and a head-up display system for the helmet.