Evaluating underground environmental safety is not an easy task as it involves many variable evaluation indexes which are immeasurable, vague and uncertain. Also, the available information for evaluation may be incomplete due to the difficulty of underground data acquisition and complexity of the influence factors. Considering the different effect of various evaluation indexes on underground environmental safety in different conditions, a dynamical weight algorithm of various indexes is presented. The fuzzy evaluation method with dynamic weight is applied in evaluating underground environmental safety at one of the coal mines in YangQuan coal industry. Findings conclude that the proposed dynamic weight method is better than the traditional method, which gives a promising prospect in underground environmental safety evaluation. And it provides a more reasonable and intelligent evaluation with accurate results.