Because stereoscopic technique can reproduce three-dimensional information, it has been widely used in various virtual reality systems. However, the visual mechanism of stereoscopic viewing is different from natural viewing, which would cause conflicts between eye vergence and accommodation and result in visual discomfort. To deal with it, this paper describes the underlying mechanism of vergence-accommodation conflict （VAC）, then analyzes the visual artifacts generated by VAC and the corresponding measurement. After reviewing recent researches on visual comfort assessment of stereoscopic content, potential objective indicators of visual fatigue are classified into optometric measures, brain activity measures and autonomous nervous system measures. At last, future research questions and potential applications of these researches on head mounted display are discussed.