The invention discloses a method for assessing a damage grade of mud spillover of a ballastless track subgrade. The method includes the following steps: 1) selecting any one workshop section of the ballastless track subgrade, determining an evaluation index, and obtaining a sensitivity factor of the evaluation index by testing and calculating; 2) determining a sensitive index; and selecting the evaluation index with largest a |Sm'| as the sensitive index; 3) establishing a mud spillover grade evaluation model, and calculating mud spillover grades of all mud spillover workshop sections; and 4)performing in-depth inspection on the mud spillover workshop sections having Gstep<1. According to the method, the sensitive index is measured, calculated and determined by selecting any one mud spillover workshop sections in to-be-evaluated subgrade sections, and the mud spillover grades are determined only by measuring the size of the sensitive index in other mud spillover workshop sections, thereby quantitatively assessing the damage grade of the mud spillover, and in combination with on-site depth investigation, qualitative assessment is carried out to form a mud spillover disease grade evaluation system, thus providing a scientific basis for a railway operation department to evaluate and rectify mud spillover diseases of the ballastless track subgrade.