A method for state estimation and risk classification of tower grounding devices is presented. Based on the grounding parameters of poles and towers in transmission line

that is

the measured value of grounding resistance and the design value of grounding resistance

the intermediate parameter is introduced

that is

σ（ the deviation rate of grounding resistance of tower from the design requirement） and γ（ the rate of change of the measured value of grounding resistance）. The statistical method with the aid of quartile method evaluates the status of tower grounding device

and divides the entire towers of transmission line into five levels of ABCDE according to the degree of risk. Under the premise of a large amount of data accumulated on tower grounding parameters of transmission line

and based on the expectation-maximization（ EM） algorithm

the distribution rule of tower grounding parameters with time is predicted

and the risk classification of tower grounding device is corrected by the predicted parameters before the measurement of tower grounding parameters.