The invention discloses a backlight module, a manufacturing method thereof, and an LCD device. A backlight module comprises an LGP; the light emergent surface of the LGP is covered with more than two material layers with different refractive indexes. In the invention, coatings of different refractive indexes are covered on the light emergent surface of the LGP, P light selectively transmitted, S light is reflected to the LGP for being reflected and converted to P light and then is transmitted again. The transmittance of the P light is increased by such repetition. The utilization rate of light energy is increased, and the brightness of LCDs is increased. In addition, that the material layers with different refractive indexes are covered on the light emergent surface of the LGP can be carried out using various processing modes, such as making general adhesive membranes in advance, or directly coating coatings onto the light emergent surface of the LGP, etc. The mode has the advantage of simple process, and is suitable for manufacturers to select the technical scheme of lower cost in accordance with self situation to reduce production cost.