Hydrological sequence length is limited, so that frequency curve parameter estimate of sampling exists error which can be effectively reducedby weighted optimum line method.Normograph length of weighted fuzzy optimization optimal line method is limited, and the membership function is not on the base of the infinite sample. The fuzzy weighted optimization optimal line method was improved by proposing half normal distribution of membership functions and using large sample statistical test method for extension of normograph. The ideal data, Monte Carlo random number and the practical sequence were used to test with umbiasedness and efficiency as evaluation index by using the method of grading method and the percentage of statistic analysis for the evaluation.The resultshows that the improved fuzzy weighted optimization optimal line has good statistical properties, high precision of optimum line. Andthe improved method can promote the fuzzy weighted optimization optimal line method to be applied in practical engineering.