The disclosure provides a method for regulating a light wavelength of a projection device. The method comprises the following steps. A single-color light source is provided and emits a first chromatic light. A phosphor layer is formed on an optical path of the single-color light source, so that the first chromatic light transmits the phosphor layer. The phosphor layer transforms a part of the first chromatic light to a second chromatic light, and emits the residual first chromatic light. The residual first chromatic light is further mixed with the second chromatic light to generate a third chromatic light. The wavelength of the third chromatic light is regulated by adjusting the proportion of the luminous intensity of the residual first chromatic light and the second chromatic light.