The invention provides a fast and accurate method for measuring free acid concentration in acid liquid. An automatic potentiometric titrator is used to measure total acidity of the acid liquid. EDTA (ethylene diamine tetraacetic acid) which prevents iron ions, aluminum ions and the like from hydrolyzing is used as a screening agent to eliminate influence of hydrolysis of the iron ions, the aluminum ions and the like to free acid measurement. The defect that pH jump of the free acid is fully or partially covered during automatic potentionmetric titration is avoided. Jump of equivalent point is increased, and accuracy of the measurement is increased. An analysis result is automatically calculated according to an input formula. Operation time from analysis to result obtaining is shortened to less than 10 minutes. In addition, the method is low in sampling amount, fine in reproducibility, and accurate in analysis results. By the method, labor intensity of analysts is reduced, and labor productivity is emancipated.