A backlight module is disclosed in the present invention. The backlight module includes a light guiding plate, a light source, a transformer and a constrainer. The transformer is disposed between the light guiding plate and the light source. The transformer is for transforming a first beam emitted from the light source into a second beam, so as to transmit the second beam to the light guiding plate, wherein a spectrum of the first beam is different from a spectrum of the second beam. The constrainer is connected to the light source for constraining a movement of the transformer. The constrainer includes a first fixing portion disposed on the light source and a second fixing portion connected to the first fixing portion, the second fixing portion being disposed on the transformer to fix the transformer relative to the light source.