The invention discloses a karst tunnel water inrush level evaluation method based on a trapezoidal cloud model under a one-dimensional X condition. The method comprises a step of constructing an evaluation indicator system for a karst tunnel water inrush risk level, classifying water inrush risk and determining a level division standard, and carrying out quantification processing on a non-quantitative indicator at the same time, a step of calculating the numerical characteristics of the trapezoidal cloud model under different risk indicators and thus completing the construction of the trapezoidal cloud model, a step of referring to and analyzing previous cases of karst tunnel construction and completing the establishment of a risk indicator system weight vector by using an analytic hierarchy process, a step of carrying out field survey and measurement to obtain risk indicator observation and measurement data and introducing the risk indicator data by using a cloud model algorithm underthe one-dimensional X condition to obtain different risk level membership degree values corresponding to indicators to complete the construction of a fuzzy comprehensive evaluation matrix, and a stepof selecting a suitable fuzzy synthetic operator and fusing the weight vector and the fuzzy comprehensive evaluation matrix to obtain a water inrush risk grade membership degree vector and determining the karst tunnel water inrush risk level.