A method for detecting environmental illumination and a fixture built-in motion sensor thereof, comprising: obtaining the environmental lux level on visible light band through a visible light detection module and the environmental lux level on invisible light band through an invisible light detection module; the lamp is turned on when said environmental lux level of visible light is lower than the preset lux level; obtaining the mixed lux level of the lamplight and the natural light on visible light band and obtaining the mixed lux level of the lamplight and the natural light on invisible light band; calculating the turn-off threshold value for turning off the lamp through the environmental lux level and the mixed lux level. The illumination of natural spectrum is detected accurately without being interfered by the illumination of the artificial spectrum emitted by the lamp itself, thus the lamp can be controlled accurately and stably.