The invention provides a predictor identification method of a medium-long-term runoff and a medium-long-term runoff predicting method. The predictor identification method comprises: step one, standardization processing is carried out; step two, a forecast period is set, a standardized runoff sequence Q and a climatic factor set sequence F that include a series of different lag phases form a candidate predictor set X and a corresponding standardized runoff sequence Q is used as a set Y in Lasso regression; step three, a parameter lambda is given, crossed verification is carried out and a prediction set Y' is calculated, the prediction set Y' is compared with the set Y to obtain a first evaluation index of the parameter lambda; step four, M different parameters lambda are selected, first evaluation indexes are normalized, and the results are added and then scoring is carried out; step five, statistics of total scores of the parameters lambda is carried out, and the parameter lambda withthe highest total score is selected as an optimal parameter; and step six, on the basis of the optimal parameter, regression coefficients of all climate factors obtained at the step three are obtained, wherein the climate factor corresponding to the non-zero regression coefficient is identified to be a predictor.