The disclosure provides a projection system and a projection method. The projection system includes a screen, a projector, a first posture sensor and a correction controller. The projector projects a display image onto the screen. The first posture sensor is disposed on the screen and configured to measure first angle information of the screen, and the first posture sensor extends along a first direction, and the first direction is parallel to a normal direction of the screen. The correction controller is connected to the projector. The correction controller determines whether to adjust the display image of the projector according to the first angle information. The disclosure further provides a projection method using the projection system. The projection system and projection method of the disclosure have lower hardware requirements and shorter correction processing time.