Panic plays a critical role in the management of emergencies, but its mechanism regarding the disaster stimuli is not yet clear. This study aims to construct a Clarity-Intensity model for evacuation panic through a VR experiment. 15 disaster stimuli are designed to build a Clarity-Intensity plane, and are applied in the virtual environment. The brain activation levels are measured with an fNIRS device with regard to every stimulus, and this metric is used in this study as a representation of panic. The brain activations are quadratically regressed on the Clarity-Intensity plane, and it is discovered 1) that the brain is more activated with higher Clarity within lower ranges of Intensity, and the opposite is applied within higher ranges of Intensity, while the activation is elevated with higher Intensity within all ranges of Clarity, and 2) that the left half is generally not less activated than the right half, and the difference peaks within medium ranges of Intensity and a high Clarity.