Abstract: Objective： To establish an volumetric method for the content determination of magnesium valproate sustained release tablets and evaluate the uncertainty in the measurement. Methods：With EDTA complexometric titration fluid of magnesium ions of test sample solution， titration end point， determined by metal indicator discoloration principle for consumption EDTA titration volume of fluid (colour from purple to blue when titration， namely for the titration end point)， to calculate titration degree valproic acid magnesium content. To build mathematical model to calculate the content， and find out the influence factors of uncertainty， and to evaluate each component. Results：The marked content for magnesium valproate sustained release tablets was 99.1%， the expanded uncertainty was 1.0% (k=2). Conclusion： EDTA volumetric method was developed for the determination of magnesium valproate sustained release tablets content of absolute error less than 0.5%， the relative error remain below 0.5%， the uncertainty of evaluation results are accurate and reliable， to provide technical guidance for the drug inspection. By analyzing the influence of uncertainty factors， affecting the main reason for the test results， improve the reliability of test results.