A focus adjustable head mounted display system used in a head mounted display device for displaying digital contents comprising: a digital content storage unit for storing digital data including audio data and image data; an signal processing unit for receiving image signals from the digital content storage unit and processing the received image signals; a focus adjustment unit for adjusting an input image from the digital content storage unit according an input focus adjustment signal so that the adjusted display image responses the focus adjustment signal; an image display unit installed at a head mounted display device for displaying the adjusted display image; a focus input unit being a manually operation device for generating a focus adjustment signal to the signal processing unit; and the focus input unit providing a focus value to the focus adjustment software. A head mounted display device for realizing the focus adjustable head mounted display system is also disclosed.