A lighting system for dim ambience has at least one light source module, implemented in an ambience. The light source module has multiple light emitting units. Each unit is respectively controlled to produce a luminance. A luminance detecting unit detects a photonic luminance and a luminance ratio. A processing and operation module calculates a mesopic luminance according to the photonic luminance and the luminance ratio. When the photonic luminance is less than a dim-light setting value, a power control information is obtained by a fitness operation with a given condition set. The power control information is corresponding to an optimized mesopic luminance after fitness. A control unit receives the power control information to modulate the luminance of the light emitting units.