A head mounted display apparatus and a backlight adjustment method thereof are disclosed. The head mounted display apparatus comprises a display module, a backlight module, an application processor, an eye image capture apparatus, an infrared (IR) light emitting diode (LED) and an application specific integrated circuit (ASIC). The backlight module provides a backlight to the display module. The eye image capture apparatus captures an eye image. The IR LED provides an auxiliary light source to the eye image capture apparatus. The ASIC calculates a current pupil size according to the eye image. The application processor adjusts the backlight intensity of the backlight module according to the current pupil size in an adjustment mode.