The present invention provides a luminance control method and a luminance control device for use with a projection system having a light source and a color wheel. The luminance control device includes a processing module, a power supply unit and a lamp driver. The luminance control method includes steps of issuing a power control signal to define a plurality of time segments in a cycle period, wherein luminance messages composed of different numbers of pulses are included in respective time segments; reading out the luminance messages included in the respective time segments in receipt of the power control signal; and adjusting luminance values of the light emitted by the light source to be projected to the color wheel according to the luminance messages.