Abstract: The traditional method used to specify the liquefaction grade may not provide the reliability information of the evaluation value from any single site for seismic design and decision-makers, which is important for risk decision and risk analysis of liquefaction hazard. Based on the uncertainty characteristics of risk evaluation on liquefaction grade and unascertained mathematics theory, a novel unascertained risk evaluation model is proposed. On the basis of a concept of unascertained number, the parameters of geological environmental system such as groundwater table, buried depth and clay particle content of soil layer, value of site standard penetration tests (SPT) and etc. are specified as unascertained parameters in order to establish a risk evaluation model. The possible interval value and faith degree responding to the liquefaction grade can be obtained through this model. This proposed method carries out directly the analysis process with all actual information of parameters. It can ensure the accumulated error to be minimized, and avoid the artificial supposition, and simultaneously give the faith degree of evaluation results. The results from the practical example show that the method used to assess sand liquefaction grade is feasible and effective, and that the results are satisfactory. This proposed method is of certain reference value to the risk analysis of other projects.