Abstract: Threat evaluation of radiation source is a multiple attribute decision-making problem, technique for order preference by similarity to an ideal solution (TOPSIS) as a classical multiple attribute decision-making method, is widely used in threat evaluation. While there are still some problems, for example, weight is set by human and subjects at perpendicular bisector of positive ideal solution and negative ideal solution can not be ranked. In order to solve the problems, threat evaluation method of radiation source based on variable precision rough set model (VPRSM) and relative entropy evaluation method is put forward. For this method, weight is firstly confirmed via data of radiation source with VPRSM; then relative entropy is applied to improve TOPSIS, and threat evaluation ranking can be finished with relative entropy evaluation method. The effectiveness and applicability of this method are verified according to simulation.