The disclosure is related to an ultrathin planar light source apparatus, comprising a LED light source and a light guide plate; the LED light source comprises a light emitting chip which is disposed opposite to a light incident side of the light guide plate; a polarizer parallel to the light guide plate and disposed between the light emitting chip and the light guide plate; the polarizer projects the light emitted from the light emitting chip toward the light guide plate. The disclosure controls the light emitted from the LED light source well to deflect within a certain range of angles such that the light completely enter into the light guide plate without increasing the thickness of the light guide plate.