Voltagesag is one of the mostperplexing power quality problems in power system. Accurate measurement of its characteristicparametersisthe primarypremiseofvoltagesag evaluation and management. However, the currentvoltage sag detection algorithm hassomeshortcomings, such aslow accuracyofparametersdetection, poorreal-timeperformanceand lack ofphase-anglejumps. Therefore, a new method ofvoltagesag measurementbased on RMS sliding window differenceoperator and samplingsequencereconstructionisproposedinthispapertoachievefastandaccuratedetectionofcharacteristicparameters. Firstly, theRMS sliding window differenceoperatorisused to obtain theinitialand end timeofvoltagesag. Next, theoriginal signalispartitioned according to the initial and end time of the voltage sag, and the window interpolation FFT algorithm is performed on the signalsindifferentsections. Then, thesamplingsequenceisreconstructedandthephaseiscorrected. Finally, characteristicparametersarecalculated, such asthedurationofvoltagesag, depthofvoltagesagandvoltagephase-anglejumps. The simulation resultsunder the conditions of simultaneous change of value and phase, differentharmonic components and differentSNR noiseshow thattheproposed method hastheadvantagesofhigh accuracy, excellentreal-timeperformance and strong robustness. The test results of the actual hardware testplatform meet the requirements of GB/T 30137- 2013, such as relative error of voltage amplitude and absolute error of phase-angle jumps are less than 0.2% and 1°, respectively, and absolute errorofduration ofvoltage sag is less than one cycle, which furtherverifies the effectiveness and feasibility ofthe proposed algorithm.