An illumination system including a laser light source, a phosphor wheel, a first light detection module, a first light sensing device, and a controller is provided. A first color region boundary is formed between the first and second color regions of the phosphor wheel. The first light sensing component is located downstream of the optical path of the phosphor wheel. During the period in which the first color region is cut into the optical path of the laser beam, the laser beam passes through the first color region and is sensed with first laser beam timing information by the first light sensing component. The controller determines a first displacement value between the first label and the first color region boundary according to the first laser beam timing information and the reference color timing information. A projection device and a color wheel calibrating method are also provided.