Quantitative determination of polyethylene glycol (PEG) is usually needed in membrane technology, water environment and chromatography. The concentration of PEG is generally measured with the Dragendorff reagent. But in the literature method, the hydrolysis of the Dragendorff reagent results in nonlinear calibration curve and unsatisfied reproducibility. In this paper, a modified Dragendorff approach was provided to inhibit the hydrolysis in the analysis. The effects of acids and its concentrations were studied. The results showed that, with the modified method, the calibration curves for PEG 4000, 6000, 10,000 and 20,000 displayed good linearity.