The invention discloses a relay protection device state assessment method based on online monitoring information. The method comprises the following steps of analyzing operation and maintenance data, selecting important quantifiable indexes as static state assessment indexes, analyzing important modules in a relay protection device, and determining important monitoring indexes as dynamic state assessment indexes; establishing a three-level assessment index system according to the data type, the internal structure and the plug-in index; carrying out deterioration degree calculation according to features of assessment indexes and completing normalization at the same time; determining the weight of each assessment index according to evaluation of an expert; determining a remark set, a membership function and a fuzzy operator; carrying out grade of membership calculation of indexes of the bottom layer, and carrying out grade of membership dynamic correction of the index deterioration speed; and establishing a fuzzy relation matrix, carrying out upward calculation step by step with the combination of weights, and finally obtaining an overall assessment result of the relay protection device. The invention establishes more perfect evaluation indexes, and carries out dynamic correction of the assessment result according to the index deterioration degree, so that the adaptability is stronger and the accuracy is higher.